You are right patty she probably would not believe her, but him and he would try to make her look like an ass, but so what? She has a right to hear it anyway.
Yes, a lot of the time women do know and just let it happen, but maybe she honestly doesn't know? I know I would want to be told, but then again I do not tolerate cheating at all.
I don't care how long I have been with someone, whether we are married/have kids or whatever, cheating is it. That is the end. That is one thing that cannot be "worked out" with me. You knew what you were doing before and when you were doing it, so to freakin bad. I do not cheat and would not cheat, so I expect the same from my man.
But, anyway, point being it is really up to you what you do. Just put yourself in his wife's shoes and figure out whether you would want to be told or not, and then decide if it is worth it to you. But, if I were her, I would want to know.
Just my opinion.