Now that I've had a Chance to sleep on this...
I don't know why this didn't occur to me before but you said that emotional abuse (from your father) has been a part of you life as long as you can remember. While it was your father who was doing the abusing, your mother bears part of the responsibility for it, too.
Let me explain --- I'm gathering that since this abuse was continuous, your mother never spoke up to your father about it? Did she ever come to your defense when this was happening? If not, did she ever explain why not?
You see, I grew up in a similar environment except it was my mother who was verbally and emotionally abusive. My father enabled this to go on --- he even threw us kids under the bus. For example, when my mother was being verbally abusive to me, I would speak up in my defense. If my father was around, he would take me aside and tell me to just ignore her because when I spoke up, she would yell at him later. He didn't like that and said that he wanted some peace in the house. Wrong answer! As a parent, his job was to protect us kids and put our well-being first.
I didn't realize at the time just how wrong that all was. It wasn't until I discussed all this with my husband that he pointed out how my father was putting himself ahead of us kids. My father had set himself up as the "good guy" and my mother as the "bad guy" when in reality, he wasn't such a good guy to us kids.
All this was driven home to me when I became a parent. Parents aren't perfect and have their bad days. Sometimes my husband would get angry at our daughter and blurt out something hurtful. When that would happen, I would speak up and tell him not to talk to her like that. Then I would tell him he owed her an apology. On the flipside, if I was being snappish with our daughter, he would point it out to me. Having grown up like we did, made me resolve to do much better when I was a parent.
I now know why it bothered me when you said your mother was your best friend. Since you know she has endured sexual abuse, that tells me that she is using you as a confidant regarding her marriage problems. Parents---no matter how bad their marriage is---should NEVER be sharing such details with their kids! I apologize if what I'm about to say sounds harsh but your mother hasn't been a good friend if she has dragged you into the middle of this!
After having said all this, of course you should encourage her to get out of a bad situation---especially if it has escalated to physical abuse. If necessary, be blunt with her and ask her to think about how you would feel if she ended up in the hospital due to it, or worse if she ended up dead.
Tell her to call the Domestic Abuse hotline and offer her a place to stay if she decides to leave. Also, tell her not to worry about dragging your father's name in the mud. In fact, I'm betting that's in part why your mother stayed with him and allowed you to be raised in such a toxic environment. A divorce can be done cleanly and simply if both parties are agreeable.
At some point---for your own peace of mind---consider asking your mother why she put you through all this as a kid and why she didn't leave your father. Hear her out. When I've brought up the topic with my father, he only has said "What else was I supposed to do?" They did separate once but got back together. He said that it would have bankrupted him to divorce. IMO, that's a poor excuse---using money or lack of it for not leaving.
Once again, good luck but this is all up to your mother. Again, encourage her to call the hotline and offer her a place to stay. However, in the end it's entirely her call---so to speak.