Haar elmboog was seer, daarom moes sy onmiddellik gaan sit.

Yes. Haar here means her.

In this sentence:

• Haar elmboog = her elbow

A useful note for English speakers: haar can look a bit like English hair, but here it is the possessive word her, not the noun hair.

Examples:

• haar hand = her hand
• haar been = her leg

Elmboog means elbow.

It is a straightforward noun:

• die elmboog = the elbow
• haar elmboog = her elbow

So the first part of the sentence, Haar elmboog was seer, literally means Her elbow was sore.

Here seer functions like an adjective meaning sore or painful.

So:

• Haar elmboog was seer = Her elbow was sore
• literally: Her elbow was painful/sore

Afrikaans often uses seer with body parts:

• My kop is seer = My head is sore / My head hurts
• Sy been is seer = His/Her leg is sore

You can think of seer as describing the condition of the body part.

Because was is the past tense of is, and the sentence is describing a state in the past.

• is seer = is sore
• was seer = was sore

So:

• Haar elmboog is seer = Her elbow is sore
• Haar elmboog was seer = Her elbow was sore

Afrikaans often uses was + adjective to describe how something felt or was at that time.

Daarom means therefore, that is why, or for that reason.

The comma separates the two linked ideas:

• Haar elmboog was seer
• daarom moes sy onmiddellik gaan sit

So the sentence structure is:

• cause: her elbow was sore
• result: therefore she had to sit down immediately

The comma is very natural here because daarom introduces the consequence of the first clause.

This is a very common Afrikaans word order question.

Afrikaans follows a verb-second pattern in main clauses. That means the finite verb usually comes in the second position.

In the second clause:

• Daarom is placed first
• so the finite verb must come next: moes
• then the subject comes after that: sy

So:

• Daarom moes sy ... = correct
• Daarom sy moes ... = not correct in standard Afrikaans

This is similar to other Afrikaans sentences:

• Vandag gaan ek huis toe. = Today I am going home.
• Toe het hy gelag. = Then he laughed.

When something other than the subject comes first, the verb usually comes before the subject.

Moes is the past tense of moet, which means must or have to.

So:

• moet = must / have to
• moes = had to

In this sentence:

• sy moes ... gaan sit = she had to sit down

Examples:

• Ek moet werk. = I must / have to work.
• Ek moes werk. = I had to work.

Yes, literally gaan sit means go sit, but in natural English it is usually translated as sit down.

So:

• gaan sit = go and sit / sit down

In Afrikaans this is a very normal way to express the action of sitting down.

Examples:

• Gaan sit asseblief. = Please sit down.
• Sy het gaan sit. = She went to sit / She sat down.

In your sentence:

• sy moes onmiddellik gaan sit = she had to sit down immediately

Because after a modal verb like moes, Afrikaans normally uses the following verbs directly, without te.

Here the structure is:

• moes
  • gaan sit

So:

• sy moes gaan sit = she had to sit down

This is normal after modal verbs such as:

• kan = can
• moet = must
• wil = want to
• sal = will
• moes = had to

Examples:

• Ek kan swem. = I can swim.
• Hy moet werk. = He must work.
• Ons moes vertrek. = We had to leave.

Onmiddellik means immediately.

In the clause:

• daarom moes sy onmiddellik gaan sit

the word order is:

• daarom = therefore
• moes = had to
• sy = she
• onmiddellik = immediately
• gaan sit = sit down

So onmiddellik is an adverb telling us when she had to sit down.

Its position is very natural in Afrikaans: after the subject and before the main verb cluster.

Afrikaans verbs usually do not change according to the subject the way English verbs sometimes do.

So you get:

• ek is, jy is, hy is, sy is, ons is
• past: ek was, sy was, ons was

And with moet/moes:

• ek moet, sy moet, hulle moet
• past: ek moes, sy moes, hulle moes

This is one of the simpler features of Afrikaans grammar: there is very little verb conjugation compared with many other languages.

Here sy means she.

That is because it is the subject of the clause:

• sy moes onmiddellik gaan sit = she had to sit down immediately

Compare:

• haar = her
• sy = she

So the sentence uses both:

• Haar elmboog = her elbow
• sy moes = she had to

That contrast is very useful to remember:

• haar = possessive/object form
• sy = subject form

It is natural Afrikaans.

A native speaker would understand it easily, and the phrasing is normal:

• Haar elmboog was seer = Her elbow was sore
• daarom moes sy onmiddellik gaan sit = therefore she had to sit down immediately

A few small variations are also possible in Afrikaans, but the original sentence is perfectly idiomatic and grammatically normal.