synthesis

1. Sentence 2 does not make any sense.
2. The phrase "going swimming" slightly idiomatic and rather clumsy in this context. I would just use the word "swimming".
3. If I were asked to produce one single sentence which conveyed the meaning of the two sentences you wrote, I would write something like this:

Shermaine would rather swim alone than [do so] with her friends.

Words in square brackets optional.

Many thanks, Contrex.

I;ve removed 'going'.

Shermaine does not like swimming with her friends. She likes swimming alone.

I believe the above sentences when joined should be as follows.

Shermaine prefers swimming alone to swimming with her friends.

Am I correct?

I believe your version is better, but is the amended version of mine correct?

Many thanks.

It is "correct" in the sense that it does not break any grammatical rules, and it conveys the meaning intended, but it commits the aesthetic sin of repeating the word "swimming" unecessarily.

Many thanks, Contrex.

I agree with what you said.