You could email him from an anonymous email account and tell him that you are the spouse of one of the women he screwed and that you are going to expose him to his wife. Tell him you are contacting him as a courtesy so that he can come clean to her before you blow it all up. Mention that this is a courtesy to her, not him, because it will be better for her to hear it from him. Give enough details that he knows you actually have the info on him but not so much that he would know that you are the ex mistress involved. That way, if he tells his wife what is going on he'll have to spill way more info than he has to because he won't know which affair is about to be revealed.

You can toy with him for a while this way so that he's always on edge 24/7. Or, he comes clean to her (which she honestly deserves anyway) and she maybe moves on to someone better and he gets screwed, but you won't have done it directly. You're just screwing with him because he's a POS.

It would be up to you if you actually ever told her, even as the supposed husband of one of his mistresses. I'm guessing you won't, but that you'd have fun making him squirm.

I'm mid way through this same scenario only in my case I'm not pretending anything and I do intend to out my husband's ex mistress to her SO. She's had countless affairs and has no idea which wife I am. It's poetic really...all the adulterous promiscuity making this a so much more complicated for her. I also threatened to contact her teenage kids (that she left in another country. ..Who does that?) I would NEVER bring her kids into this but she can feel free to worry about it.

When I contact her SO about it, it will be with nothing but kindness and concern, as I know it will hurt, but he deserves to know that she's with him for the mansion and lifestyle and brings other men to his home to fuck while he's at work. I really hope she'll hate herself and have to move out of her luxurious life, and that he'll find someone much much better.