I'd do the same and tell him. Odds are that she'd did it to him and she'll do it to you.
I wouldn't be too mad unless I've spent a few years with her or something. I'd just say f it and walk away.
I don't get why people cheat in the first place. They could just leave. I don't know if they think they're doing something good by not hurting the others' feelings or something, but cheating on someone is worse than just leaving, in most situations.