[Always *]

I agree with PSY, I don't think you should just up and tell her, even if he claims she already knows. He might perceive it as a betrayal because you didn't let him handle it or what ever. It might not be rationale because she does deserve to know, but it might make it harder to recover from if she heard it from a stranger.
Any how, I'd talk to him first, tell him that it upsets you he cheated especially because you've been cheated on and let him know how it hit home for you and that it isn't sitting right with you that you (at least think) that she has no idea what's going on and that if you were in her situation you'd want to know etc. If you think you'd forgive him for it as long as he was truly remorseful and regretful etc. then let him know that, it might help him be honest if he knows it's not a relationship ruiner. You might want to make it abundantly clear that while you don't want to step on his toes you are seriously considering telling her if he doesn't do it himself.