Most Helpful Girl
If I were her, I'd want to know, personally.
I was in a situation with my one friend that was in a temporary long distance relationship with her boyfriend and I found out he was cheating on her. I was torn, but I eventually told her because I felt she deserved to know (she was moving back soon to be with him). It turns out, other people already mentioned his cheating to her, but she didn't believe any of it : /