So she cancelled her appearance at our marriage counseling appointment today.
I'm wondering if I should contact the dbag's soon to be ex-wife now?
What do you hope to achieve? If they truly are headed toward divorce, then there is probably no point--she won't have any leverage. If, on the other hand, he has been lying to your wife and his (cheaters lie) about the status of his marriage, then his wife should know.
The question with any of these "tough love" steps always comes down to the motivation behind them. If you are trying to preserve your family, you can make a case for any of the steps you have taken. If you are angry about your wife's betrayal and trying to punish her, then you should think about them more deeply. Given the brevity of your posts here, I don't have a feel for it either way.
