You can't control your mother only you. so if she wants to confront an abuser then she is going to confront an abuser.

But you can decide if you want to be a part of your mothers confrontation of the abuser or if you want to confront the abuser yourself.

If your mother decides to confront your abuser there may be fall out to you because of it. So you can put in place safety protocals and so on with your therapist or alone.

for example when my abuser realized I had gone public some of the fall out went another relatives way just because her and I were close and they felt she should have known what I was doing. (as if her and I are supposed answer to each other for everything we do!) anyway she protected herself by saying - look what she does is not my business she is an adult. you want to discuss this discuss it with her. then she either walked away from them or hung up the phone on them. When the fall out of threats and so on came back on my friends they threatened right back with don't bother me again or I will arrest you for harrassment and take out a protection order against you. you have a problem with so and so take it up with so and so leave me out of it or find yourself sitting in jail.

in either case they soon learned if they had a problem with my confronting and going public they take it up with me because I was the one confronting and going public. And my reaction - you want to fight about this fine see you in court. and then stayed with friends so that they could not get me alone without a witness to their threats and so on.

You cant stop your mom. your mom controls moms behavior but you can protect yourself. Follow through with talking to your therapist and set things up so that you will be safe in the event that mom follows through and confronts the abuser.