What about if MIL emails her and says yes, he's a cheater? I think there's a crack in the facade and maybe the doubt would be that much more if his own mother backs up my story.
You're assuming she (OW) doesn't know the truth. Don't be so quick to make that assumption.
Just as (more?) likely, she KNOWS what your husband is, and just isn't willing to face it. After all, he's cheating with HER, isn't he? Doesn't that pretty much MAKE him a cheater already?
Look, don't get me wrong -- I'm ALL ABOUT controlling the endgame. But all you can do is paint the corners of the box for the other people involved; THEY have to make their own decisions within that. Otherwise, it'll drive you batty.
