WuzLovesDubs: If you're resolute to leave him, please don't let him push your buttons. I thought: If he and the other "sister" happen to get it on because of the emotional intimacy they seem to be sharing, then he's the one who's going to be before the elders explaining himself, plus he'll drag her down with him for what happened. Either way, whether he does it or not, it shouldn't make any difference to you. I know it's easier said than done, but nothing he does further should change your decision.
Then I thought: No. She needs to be prepared. Here's why: I knew a guy who worked at the same compony I did and his girlfriend, a nice lady. After my wife and I became friends with them as a couple, I learned that she used to be a Witless. What happened was that her husband at the time, also a Witless, had an affair with her mother. Holy crap! I had never personally heard of such a thing before. In an ironic twist, the husband begged forgiveness to the elders. The elders turned and asked my friend to come to a meeting, I guess with the intent that since the husband asked Jehovah for forgiveness that she would forgive him too. When she refused, they disfellowshipped her instead. My understanding is that he and her mother are still in the congregation and she's out.
So, it's possible that from your actions, you'll get some backlash. So be ready. Let him eat his own vomit. I'm speaking metaphorically.