I don't think you should tell her. I agree that confronting HIM is the best approach, since he's the guilty party for allowing it to happen. He can then choose to tell his wife or not, but I don't think it would benefit anyone except maybe yourself to inform her of this. She will be hurt and it will cause an issue for them, which is never a good idea to be the cause of. So I would confront him, and let him know how it also makes you feel, knowing this about him when she does not.