As a friend, all you can do is be there. Trust me I know what you are talking about. When I was a lot younger, my sister was in a horrible relationship. Although his true colors did not come out until after they were married. They had three children together. My sister was not allowed to speak to any of her family. Her husband filed assault charges against my brother even though my brother was on a tug boat hundreds of miles away at the time all because we attempted to make contact. My sister called out of the blue when we had not spoken to her in nearly a year in secret. A week later an institution called saying my sister needed to speak to her mom and dad. Her husband had drugged her and called the cops. She fought for almost 2 years to divorce the man and get her kids back, while he refused to let her have contact with the children. She unfortunately passed away from a blood clot in her heart, but she was still fighting for divorce and custody.
I hope your friend eventually wakes up and sees the toxicness of her relationship. But in the meantime, realize that sometimes people are blind to the horribleness of a certain relationship.