Then simply tell her "when you have dinner with my wife and another man, it hurts me deeply. I love your daughter, but I don't agree with what she is doing, and I feel you enable her when you do that. If you continue to support their relationship, it's going to put even more distance between you and me, and that's regrettable." -- or some such.
Puppy
These points above are excellent... try not to attack or speak down to her mother.. you want to help her understand your commitment, and ask for her help in supporting your marriage and in supporting family therapy...
The OM will just look like a sleazeball when you do that.. which he IS
