Alcohol can turn the best person to evil. Perhaps your mother needs Al-anon, so she can better understand the disease of alcoholism. Then she can make her own decision to leave or not.
My father was an alcoholic and fell off the wagon numerous times. My mother married him 3 times before he married someone else and then passed away. The fit of rage in an alcoholic is incredible and then flip the coin and they are the best of people. I have been married to my husband for 16 years yesterday and he is a recovered alcoholic. I never knew him as a drinker or otherwise, but he has shared it was ugly. AA helps my husband cope with what would be some ugly situations to this day. Recovery without help is a difficult route, although not impossible. If you don't already know, once an alcoholic, always an alcoholic.
I have never attended Al-anon and I know it is not for everyone. I am a reader and researcher (self proclaimed) and read until I feel I have a good understanding of situations. So I don't attend, but feel I get help from my own research.
She may not leave him for good and and that may work for them, but she should learn about the person she is with and understand them.
At the same time, I will point out that the reason he is able to take on his controlling personality is because she allows him to. She hasn't seemed to be independent at any point, so it is likely he has controlled her for most of their relationship. She could probably use a little help with this herself. You can only suggest family counseling to her and it will be up to her to take that advice.
Best wishes.