Learn Hardcore Russian: Моя подруга тоже стала моим близким приятелем по работе. - My female friend has also become my close buddy at work.

Он стал врачом. – He became a doctor.
Она стала директором. – She became a director.
Моя подруга стала моим близким приятелем. – My [female] friend became my close friend/colleague.

So мой → моим, близкий → близким, приятель → приятелем are just the instrumental singular masculine endings.

Why is приятелем masculine when we are talking about подруга, a female?

In Russian, grammatical gender and the real person’s gender can differ.

подруга – explicitly a female friend (feminine noun).
приятель – a male-gender noun, but it can refer to a friend of any real-world gender in some contexts.

So grammatically you are saying:

She (подруга, feminine) became my close work friend (приятель, masculine noun).

Russian allows this because you are focusing on the role (приятель по работе, work friend / work buddy), not on biological gender here.
Stylistically, though, many native speakers would more naturally say:

Моя подруга тоже стала моей близкой подругой по работе.
(keeping everything feminine: моей близкой подругой)

The original sentence is grammatically correct but a bit stylistically odd, because it mixes подруга with a masculine приятель for the same person.

Why is приятелем and not приятелью, since it is a friend who is a woman?

Because приятель is a masculine noun by form, and its instrumental singular ending is -ем:

приятель → приятелем (masculine, instrumental singular)

If you want a clearly feminine form, you should change the noun, not the ending:

подруга (feminine) → подругой (instrumental)
Моя подруга стала моей близкой подругой по работе.

You cannot make приятель grammatically feminine by giving it a feminine ending like -ю (приятелью is incorrect).

What is the difference between подруга, друг, and приятель?

These three all mean some kind of friend, but with different nuances:

подруга
- Female friend.
- In modern speech it can also mean girlfriend (romantic partner), depending on context.
- Always feminine.
друг
- General word for friend (usually close friend).
- Grammatically masculine, but used in generic sense too: мой друг can be male or female in some contexts (but often understood as male unless clarified).
приятель
- Slightly weaker or more casual than друг, often like buddy, pal, acquaintance-friend.
- Typically not as emotionally strong as близкий друг.
- Grammatically masculine.

In the sentence:

Моя подруга – [female] friend in your life.
моим близким приятелем по работе – a close work friend / colleague buddy.

So it suggests: my (female) friend from outside work also became my close friend at work.

Why is the adjective близким and not близкий in моим близким приятелем?

Because the adjective must match the noun приятелем in case, number, and gender.

Noun: приятель (masculine singular) → instrumental: приятелем.
Adjective: близкий (masculine singular) → instrumental: близким.
Possessive: мой → instrumental: моим.

So all three words align:

мой близкий приятель (nominative)
моим близким приятелем (instrumental)

This is standard Russian agreement.

Why does Russian use the instrumental case with стала?

The verb стать (past feminine стала) normally takes a predicative noun in the instrumental case when you mean to become [something]:

Он стал учителем. – He became a teacher.
Она стала актрисой. – She became an actress.
Моя подруга стала моим близким приятелем. – My friend became my close friend.

This is a general pattern with:

быть (to be) – especially in past/future:
Он был врачом. – He was a doctor.
стать (to become):
Она стала директором. – She became a director.
являться (to be, to constitute, formal):
Он является моим другом. – He is my friend.

So моим близким приятелем is instrumental because it’s the complement of стала.

Why is it стала, not становилась or something else?

Стала is the perfective past form of стать and describes a completed change: she became.

стать – perfective: result, single event
- Она стала моим приятелем. – She became my friend (and now she is).
становиться – imperfective: process, repeated action
- Она становилась всё ближе. – She was becoming closer.
- Она долго становилась моим настоящим другом. – It took her a long time to become my real friend.

In your sentence, the idea is that there was a clear change of status: at some point, she became this close work friend, so стала is natural.

What exactly does по работе mean here? Is it at work or something else?

По работе can be translated roughly as:

work-wise
in terms of work
in the context of work

In this sentence:

моим близким приятелем по работе ≈ my close friend at work / in a work context.

Nuances:

по работе – emphasizes the type of relationship: a work friendship, not necessarily outside work.
на работе – literally at work, more about the place/situation.

Compare:

Он мне помогает по работе. – He helps me with work-related things.
Мы познакомились на работе. – We met at work (physically there).

Your phrase is more like a close work friend / close colleague-friend than simply a friend who happens to be at the office.

Could we say на работе instead of по работе here?

You could say:

Моя подруга стала моим близким приятелем на работе.

This would be understood as My friend became my close friend at work, focusing more on where this friendship formed.

But по работе is slightly more idiomatic for the idea of a work-related or professional kind of friendship. It hints that the friendship is in the sphere of your working life. So:

по работе – by function / sphere (work friend).
на работе – by place / situation (friend at the workplace).

Both are correct, but the nuance is a bit different.

What does тоже do here, and where can it be placed?

Тоже means also / too.

In Моя подруга тоже стала моим близким приятелем по работе, the default reading is:

My (female) friend also became my close work friend.

The main placement is right after the subject:

Моя подруга тоже стала...

This typically means: someone else had already become this, and she also did.

Other placements:

Моя подруга стала тоже моим близким приятелем по работе.
- Still possible, but the sentence sounds less natural; the focus slightly shifts to the role rather than the subject.
Моя подруга стала моим близким приятелем по работе тоже.
- Sounds awkward; тоже at the end is rarely natural in such sentences.

Best is the version given: Моя подруга тоже стала...

Why is the possessive моя in the beginning but моим later?

Because each мой/моя must agree with a different noun in a different case:

Моя подруга
- подруга – nominative feminine singular.
- So моя – nominative feminine singular.
моим близким приятелем
- приятелем – instrumental masculine singular.
- So моим – instrumental masculine singular to match приятелем.

They are two separate noun phrases:

Моя подруга – subject.
моим близким приятелем по работе – complement in instrumental.

The change моя → моим is purely grammatical agreement.

Is there a more natural way to say this without mixing подруга and masculine приятель?

Yes. Native speakers would more often keep the gender consistent:

Using подруга throughout (very natural):
- Моя подруга тоже стала моей близкой подругой по работе.
  My [female] friend also became my close friend at work.
Or using more neutral коллега (can be any gender):
- Моя подруга тоже стала моей близкой коллегой.
  My friend also became my close colleague.
Or using друг in a gender-neutral function (still grammatically masculine):
- Моя подруга стала моим близким другом по работе.
  This is closer to your original but avoids приятель. Still, there is the same grammatical-gender mix, though it feels a bit more natural than приятель to many speakers.

The given sentence is correct but sounds slightly unusual stylistically.

What is the basic word order and can we change it?

The sentence:

Моя подруга тоже стала моим близким приятелем по работе.

Basic structure:

Моя подруга – subject.
тоже – adverb (also).
стала – verb.
моим близким приятелем по работе – complement in instrumental.

Possible variations:

Тоже моя подруга стала моим близким приятелем по работе. – Sounds odd, heavy emphasis on тоже.
Моя подруга стала моим близким приятелем по работе тоже. – Feels unnatural; тоже is not usually placed at the end here.
Моя подруга стала тоже моим близким приятелем по работе. – Grammatically OK, but most people prefer тоже after the subject.

So in practice, Subject + тоже + verb + rest is the most neutral and common pattern.

Is близким приятелем as strong as близким другом?

No, близкий приятель is usually a bit weaker or more casual than близкий друг.

близкий друг – a very close friend, strong emotional connection.
близкий приятель – close buddy / close pal, but can sound slightly lighter, not necessarily as deep as друг.

In a work context:

близкий приятель по работе often sounds like a very friendly, maybe slightly informal colleague.
близкий друг по работе might suggest a deeper, more personal connection that just happens to have started at work.