How do I help my friend leave an abusive new bf

What's going on - you don't give details.

What's the story behind the "abuse"?

I ask only because my grandson tried to prevent his GF from driving when she was really drunk. He took the keys away from her, she attacked him. He restrained her, she called the police. She got hysterical in front of the cops. Her mother had to come get her. No action from them against my grandson.

Another young man simply pointed his finger at his wife and she called the cops.

On the other hand, if he is taking out his bad day on her, that's another thing.

Has she been in other abusive relationships? Does he have this reputation from other exes? What do you know about this guy?

You are right- here is more details. He has a mental disorder and not taking any meds . I heard he is seeing a shrink once in a blue moon. He is getting angry very fast with her and snaps at her and she feels uncomfortable about that. When she tried to break it off, he got physical with her - pinned her against a wall and tried to force himself on her. She decided to spend the night anyway and the second day broke up with him again. Now she is telling me that she is giving him another chance. I am desperate to figure out the best solution here. I know from personal experience that someone who is comfortable acting like that in the first month of dating won't be shy to to do it again later on. She won't listen to me tho

Because she may think (rightly or wrongly) that you have an agenda.

Try appealing to her folks to talk to her.