These things progress to this point because so many of you get bad advice and sit back and watch affairs blossom whilst doing nothing. That's the worst possible thing you can do. It makes fixing it at this point almost impossible. Not impossible, but almost impossible.
Allowing fairs to continue unchallenged is never ever ever a good idea. He's her boss? That affair was easy to nip in the bud. Inform HR. Companies have very very strict policies about such things. Especially employer/employee affairs. If it goes south they can get sued by your WW for sexual harassment and she'd win. Still, at this point you can try DB'ing but honestly you let it get away from you probably too far. My advice is quit worrying about the dying M and focus on your own future post-M.
