I live in a fault state, and adultery is grounds for a fault divorce. Since they're living together, I think it should be fairly easy to prove...
OK, the fact that it's a fault state still does not mean that adultery is worth anything to you in a D. If it is worth something, and you want to get D, then proceed.
But I can pretty much tell you that if they are living together now, that his name on a D petition is not going to "scare him off". He doesn't give a rat's a$$.
Again, trying to play mind games with AP is a waste of your time. Don't waste your time.