В выходной она может отдыхать сколько угодно.

Yes. Выходной is originally an adjective meaning day-off or non-working, but in everyday Russian it is very often used as a noun-like word by itself.

Here it stands for выходной день = a day off.

So:

• в выходной = on a day off / on the day off
• full version: в выходной день

This kind of shortened expression is very common in Russian.

Because this is a time expression, not a location.

After в, Russian can use different cases depending on meaning:

• в + accusative often means movement into something, or is used in many time expressions
• в + prepositional usually means location, in or inside

Here в выходной means on a day off, so Russian uses the accusative.

Since the hidden noun is день (masculine, inanimate), the accusative singular looks the same as the nominative:

• выходной день
• в выходной день

That is why you see в выходной.

Not exactly.

В выходной is singular, so it usually means:

• on a day off
• on her day off
• on a weekend day in some contexts

If you want on weekends or during the weekend more generally, Russian more often uses:

• в выходные = on weekends / during the weekend
• на выходных = colloquial over the weekend

So в выходной sounds more singular and specific than в выходные.

Like English can, может can cover more than one idea.

In this sentence it most naturally means something like:

• she can
• she is free to
• she is able to

It does not have to mean literal physical ability. It can simply mean that nothing prevents her from resting.

So depending on context, она может отдыхать could suggest:

• she has the opportunity to rest
• she is allowed to rest
• she is able to rest

Russian often leaves that nuance to context, just as English does with can.

Because отдыхать is imperfective, and that fits the idea here better.

• отдыхать = to rest, to be resting, to relax, to rest for some time, in an ongoing or general sense
• отдохнуть = to rest and complete that action, often with the idea of getting some rest or resting successfully

With сколько угодно = as much/as long as she wants, Russian normally prefers the imperfective, because the focus is on an unbounded activity.

So:

• может отдыхать сколько угодно = she can rest as much/as long as she likes

If you used отдохнуть, the meaning would shift a bit toward a single completed result:

• она может отдохнуть = she can get some rest / she can rest once and be done

Сколько угодно is a fixed expression meaning:

• as much as you like
• as many as you like
• as long as you like

The exact English translation depends on the verb and context.

Here, with отдыхать, it means:

• as much as she wants
• for as long as she wants

Literally, it is something like however much is pleasing/desired, but you should learn it as a whole expression.

Examples:

• Ешь сколько угодно. = Eat as much as you like.
• Жди сколько угодно. = Wait as long as you like.
• Бери сколько угодно книг. = Take as many books as you like.

Because Russian often leaves possession unstated when it is obvious from context.

So в выходной can naturally mean:

• on a day off
• on her day off

If you want to make it explicit, you can say:

• в свой выходной = on her own day off
• в её выходной день = on her day off

But in many sentences Russian does not need that extra possessive word.

No. Russian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

The sentence given:

• В выходной она может отдыхать сколько угодно.

This puts в выходной first, so it sets the time frame right away: As for her day off...

Other possible orders include:

• Она может отдыхать сколько угодно в выходной.
• Она в выходной может отдыхать сколько угодно.
• Она может в выходной отдыхать сколько угодно.

All are understandable, but they sound a little different in focus.

The original version is very natural because it starts with the time expression and ends with сколько угодно, which is a nice place for the key idea as much/as long as she wants.

Sometimes yes, but not always as naturally in isolation.

Russian often drops subject pronouns when the subject is clear from context. So in a conversation, you might say something like:

• В выходной может отдыхать сколько угодно.

If everyone already knows who is being discussed, that can work.

But as a standalone sentence, keeping она is clearer and more natural:

• В выходной она может отдыхать сколько угодно.

You could also make it impersonal:

• В выходной можно отдыхать сколько угодно. = On a day off, one can rest as much/as long as one likes.

Yes, absolutely.

• В выходной она может отдыхать сколько угодно.
• В выходной день она может отдыхать сколько угодно.

Both are correct.

The version without день is simply more compact and very common in normal speech. Adding день can sound a bit fuller or slightly more explicit, but it does not change the basic meaning much.

A natural stress pattern would often be:

В выходно́й она́ мо́жет отдыха́ть ско́лько уго́дно.

Main stresses:

• выходно́й
• она́
• мо́жет
• отдыха́ть
• ско́лько уго́дно

In real speech, the strongest stress often falls on the information the speaker wants to highlight. For example:

• if the important point is when, stress в выходно́й
• if the important point is how much, stress ско́лько уго́дно

So intonation can shift depending on context, even though the words stay the same.