She needs to arrive at the conclusion to leave him herself.
The more people tell her to leave him, the more she will stick up for him and stay with him. She then gets stuck in the us vs the rest of the world trap. When in the trap its even harder to leave because she's told herself the rest of the world is against her (not true).
Next time she calls instead of offering her advice or suggestions ask her what SHE is going to do about the situation. Help her work through her options and ask her what the consequences are of staying vs leaving.
She also needs professional help, suggest that you may not be the best person to speak to and refer her to the professionals. That in the capacity as a friend you will be there but not as a counsellor.
Until she is ready to leave him, there isn't much anyone can do. Domestic abuse is one sick cycle.