Im my mom's best friend... honestly her only friend.
My dad was controlling and she wasn't allowed any friends. I know as much about my parents sex life as I do my own! She has nobody else to talk to... so for those saying she shouldn't be talking to you about it.. maybe not... but I get it.
I would really encourage your mom to get into counseling. She needs it for herself. They will help her find ways to talk to your dad about what is going on. The will help her find her strength that she has hidden inside of her. They will help her stand on her own two feet instead of being a door mat.
All of this didn't happen over night so it will take some time for her to find herself again.
Once my mom found her strength she also decided that she wasn't going to live like this anymore after 32 years and she moved out. My parents divorced earlier this year after 34 years of being married.
Ironically it was harder on my dad figuring out how to do it all on his own and how much my mom actually did that he never seen while she was there. It is taking some time for both of them, but over all they both are happier now. Now that they are happier away from each other I have been able to have a greater deeper relationship with both of my parents that wasn't there before.
Im not telling you to have your parents divorce, hopefully once she starts standing up for herself things will get better for them.. but if they are anything like my parents, it got worse when my mom stood up for herself. Divorce was the best thing for both of them. Its amazing how much both of my parents have changed for the better.
Good luck!