Personally, I see it as cake eating. I don't think he will possibly be able to see any changes in you with the OW around or involved. I would let him see what it's like to not have you in his life. That is what he chose when he left. Be strong and be nice about it. He'll balk and say that he was thinking about coming back and blah blah blah. He's not thinking about coming back if there is OW to fall back on.
"It is excruciating pain. It is the pain of separation, the pain of loss, the pain of dreams and expectations unrealized. It is the loss and death of a mirage."
