Pronoun references are proper unless you don't have ambiguity.

The MGMAT sentence example is right as 'they' can refer only to 'guests' in the sentence.

"When the guests finished their soup they were brought plates of salad".

Whereas in your example 'they' can refer to both 'guests' and 'friends'. There is ambiguity.

"When the guests finished talking to their friends they were brought plates of salad".

So deciding right or wrong depends entirely on the antecedents you have in a sentence. If a particular pronoun has only one antecedent then it is proper, if it has more than more antecedent which it can refer to then you should avoid that option.

In the Pronoun section of Manhattan GMAT Sentence Correction book.
The following statement is given as correct.

"When the guests finished their soup they were brought plates of salad".

Here "they" has the antecedent "guests"

But I still feel it is improper, for example, using a similar structure

"When the guests finished talking to their friends they were brought plates of salad".

Here, they can also refer to friends? cant it.



Yes absolutely, in the second sentence 'they' is ambigious as 'they' does not have a clear referent.....

you are correct in that aspect...