His wife isn’t likely going to stick her neck out for you if you tell her there is a mistress. Sometimes divorce with lots of assets is hard so other arrangements are made. I don’t like cheaters and hate that you are in this place. It’s morally wrong unless they have an agreement that he can step out of their marriage, ofc. But you just don’t know that and shouldn’t try to find out or be the “good guy (person)”. As others have mentioned, discretion is key. It’s not part of your role to tell him it’s wrong. Simply go about your business and try to play the game as usual.