How does someone escape from an emotionally abusive relationship?

I have a friend, a guy, who is married but in an emotionally abusive relationship. His wife is controlling and abusive and he is little more than her personal slave. Doing all the work at home, cooking and cleaning and paying all the bills. He works many hours for his wife's business but gets nothing in return and he is obviously always broke.
He loves her but I know he is desperately unhappy at this treatment. What should he do, and how can he start again ? What advice can you give him please ?