How to save my mom from my alcoholic and maniac dad.

Hi sirs, My dad has been an alcoholic his whole life. My mom has suffered for more than 25 years now. He was in military and gets his pension since he retired 8 years ago. But now for last couple of years his behavior has become much worse. He won't seek any professional help. He drinks every day and the words that come our of his mouth are unspeakable. He is so loud we have to change houses every now and then because the neighbors complaint about his foul language. And for the last 2-3 years he has started to hit my mom. It's when I'm not home usually. He even hit her in front of me many times but I can only stop him for now. Can't hit him back because he is my father. Both side of our mom's and dad's families know his issues. Dad's family actually hid the fact that he was an alcoholic before their marriage. Mom has already had an car accident many years ago and is very weak now. She can't take anymore of his mental and physical torture. I have to go to work and my only brother works abroad. So how can we finally end this. Mom has agreed to this but we thought we shouldn't ask our local legal opinion first. Otherwise dad will know it somehow and it will become a huge problem. Regards, Tom