The narrator is complaining about men on the train making noise and says:
I'm uncertain of the interpretation of に together with くれる here. I can see two possibilities:
a) に translates as to. She waited for the conductor to give a warning to the men and that would benefit her. (I think this is more likely)
b) に translates as for. She waited for the conductor to give a warning for the men and that would benefit the men (because it will improve their character or it will stop someone going and punching them for example).
Are both my interpretations grammatically valid (with and without the context) or have I failed to appreciate a grammar point which makes one of them obviously wrong? Thanks.
Edit: Perhaps I should explain further. I'm have little doubt that a) is the correct answer, but I'm interested to know if b) is a grammatically valid interpretation. For example:
My father bought a camera for me.
This sentence has the same structure but the person receving the benefit is the one marked by に.