If I were you, I would just go ahead and tell her and present her with proof to back it up. It’s likely she won’t believe you unless you show her proof of it, if she trusts her man. Speaking from experience, I once had a boyfriend that cheated on me with about 8 different girls that I know of, and I didn’t even believe my close friends at first when they told me, but I finally believed it when I got him to admit it. So proof helps a lot in that case.

At any rate…. I think most people would want to know if their significant other were cheating on them, because, not only is it a betrayal, but if he’s sleeping with other women and not using protection with them or her, it invites STDs. So telling her would ultimately be doing her a HUGE favor, even though it will most definitely make her upset… but her being upset is ultimately the result of HIS actions, not the result of you telling her about his actions. Know what I mean?