mel,
It's a difficult situation. I know it hurts and it's painful but to encourage your mom to continue living in an untrusting marriage is like asking her to endure a life long torture. Everyday her mind will wonder who he is with...constantly. It's time she move on, you move on, and he move on. This would be the ultimate message for your father, for your mother to leave officially. He will realize there is negative consequences to negative acts, if she was to stay and tolerate it he would never realize that it is bad.

I know this sounds judgemental, but I've learned through the relationships my mother, father, and myself had andone thing I know is...once a cheater always a cheater. I believe that honestly.

I'm sorry your going through this, I recommend sticking with your mom for not only her support but your own as well.