Keep pushing the "You should marry her." Tell her that she's the woman of his dreams, he was willing to give up his family, his wife, his house, even being a full-time dad to his child for her then he has found his "true love" (gag me!) and needs to marry her and spend the rest of his life with her.....

Also, maybe the anti-DBing (telling him what an immoral, horrible role model he is, and how this is going to make his daughter think it's okay to do this stuff) will push him out of the house.

There is no arriving, ever. It is all a continual becoming.