R.K.
I think it's time you looked at some real, hard facts. First of all Mary is not the home wreaker your husband is--he was married and decided he needed a side dish. She was available so he went for it. You don't know what kind of promises he made to her in the beginning. I promise I will get a divorce and marry you and we will raise my daughter together. My wife is ..... Then reality hit him she's unstable, a stalker, abusive and controlling. Ooops, maybe if I ask wifey for forgivness she will forgive and forget and everything will get back to normal and Mary will move on to someone else. You are still referring him as your husband are you still married to him? Separated or divorced?
If you are divorced and he has court ordered visitation there is nothing you can do to stop Mary from spending time with your daughter. If she really is his girlfriend he has the right to spend time with her.
Unless you have documentation of her stalking or other unstable behaviors and can get a restraining order if he wants to have her in his home when his daughter is there, he can.
I don't think she will be all that nice to your daughter. I think she is very unstable. I also think she has many classic behaviors of an incest or molestation victim.
I would not be comfortable having her be a part of my child's life either but unless you can go to court and PROVE her unfit to spend time with your daughter there is little you can do. Remember in court it is not what you think or even know but what you can PROVE.