I know this post is a little old, and I also realize my situation isn't as complicated or as serious as yours, but I do understand the feelings you are having.

My fiance has an ex girlfriend he dated in high school (I know, HIGH SCHOOL). But she treated him very badly. Practically taking his virginity, convincing him to lie to his mother, getting him drunk and high, etc etc. I realize he did make his own choices, so she's not completely to blame, but I'm just trying to show what kind of person she was and how awful she was to him. Unfortunately for me and him, his parents love her. Even today, LOVE her. (At his sister's wedding 3 years ago, she was introduced as "This is our son's ex girlfriend, but we love her!" I have never been introduced like this). She was a German exchange student going to our high school, but even today, 7 years later, whenever she's in the states, she gets together with my fiance's parents. We had the good luck to visit a week or two after she did, and practically the first thing both his parents said to us was "Oh, we got lunch with K last week!" It's very, very frustrating to both of us (more so to me...)

But what my therapist helped me realize is that in this case, it's really about boundaries. My fiance's parents don't see any problem getting together with this girl. In fact, it's spread to the whole family. An aunt that she's met maybe once likes her posts about visiting and comments and such on facebook. Neither the ex nor the family understand boundaries, so we have to be the ones to put them firmly in place. It's like Leed said, you have to take back your power. Both me and my fiance blocked her on facebook completely so she'll never show up on either of our pages again. Also, when his parents brought her up, my fiance said that we don't care and don't want to know anything about her. If I were you, I would tell your mother it bothers you if she gets together with him, since she did ask your opinion, but still letting her know that it's her decision what kind of relationship she wants to have with him. I would also tell her that if she does decide to get coffee with him, ask her not to talk about him with you or you with him. Try to figure out what boundaries you are comfortable with and then work on enforcing them. My therapist has been very helpful with this thorn in my side.

Good luck and remember to take care of you!!