يدخر صديقي المال ثم ينفق بعضه.

Because this is a normal verbal sentence in Arabic. A very common word order is:

verb + subject + object

So:

يدخر صديقي المال
= saves my friend the money / my friend saves money

In natural English we do not keep that order, but in Arabic it is completely normal.

You could also say:

صديقي يدخر المال

That is also correct, but it feels more like you are putting extra focus on my friend.

يدخر means he saves or he is saving.

Grammatically, it is:

• 3rd person
• masculine
• singular
• present/imperfect tense

In fully vowelled Arabic, it is usually written يَدَّخِرُ.

So the subject built into the verb is already he.

Because Arabic verbs already contain that information.

يدخر already tells you:

• the subject is he
• singular
• masculine

So Arabic does not need a separate هو here.

That means:

• يدخر صديقي المال = natural
• هو يدخر صديقي المال = not the normal way to say this sentence

If you use a separate pronoun in Arabic, it is usually for emphasis, contrast, or a different sentence structure.

Because the ending -ي is an attached pronoun meaning my.

So:

• صديق = friend
• صديقي = my friend

Arabic usually shows possession by attaching a pronoun to the noun, rather than using a separate word before it.

Arabic often uses الـ with nouns in a more general way than English does.

So المال can mean:

• the money in a specific context
• or simply money as a general thing

Here, even though it literally looks like the money, the natural English translation may just be money.

This is very common with mass nouns and generic nouns in Arabic.

ثم means then or after that.

It shows that one action happens first, and the next action follows it in sequence.

Compare:

• و = and
  It simply connects two things.
• ف = so / then with a sense of quick or immediate sequence
• ثم = then / afterwards, often with a clearer sense of separation or later sequence

So in this sentence, ثم tells us:

He saves money, then later spends some of it.

ينفق means he spends or he is spending.

It is also:

• 3rd person
• masculine
• singular
• present/imperfect tense

In fully vowelled form, it is يُنْفِقُ.

It is the present tense of the verb أنفق.

بعض means some or part.

The ending -ه means his / its depending on context.

So:

• بعض = some, part
• بعضه = some of it / part of it

Here, -ه refers back to المال.

So the meaning is: he spends some of the money

Because it refers back to المال, and المال is grammatically masculine singular.

So Arabic uses the masculine singular pronoun:

• -ه = it / his for masculine singular

If the noun being referred to were feminine, the suffix would be different.

A fully vowelled version is:

يَدَّخِرُ صَدِيقِي الْمَالَ ثُمَّ يُنْفِقُ بَعْضَهُ

A few useful notes:

• يَدَّخِرُ = present tense, ending in -u
• صديقي = the subject, my friend
• المالَ = direct object, so it is accusative
• ثُمَّ = then
• يُنْفِقُ = present tense, ending in -u
• بعضَهُ = direct object of ينفق, so it is also accusative

In normal everyday Arabic texts, those short vowels are usually not written.

It can be either in Arabic, depending on context.

The Arabic present/imperfect often covers:

• he saves
• he is saving
• he usually saves

In this sentence, the most natural reading is habitual or general:

My friend saves money and then spends some of it.

So it sounds like something he tends to do, not necessarily something happening at this exact moment.