i had a friend once in a relationship like that, and she broke up with him after ages of feeling guilty. she just got sick of it one day.
i wish i'd told her more often what a sleaze bag he was so that she'd dump him sooner.
You should tell her that -for her own good- she needs to end things with him. point out to her that she CAN live without him, in fact she'll live BETTER without him, seeing as she was perfectly happy (i assume) before he came along. i'm thinking maybe she's only happy with him because he's comfortable. she's gotten used to his being around and feels safe having a boyfriend, and maybe his "intenseness" makes her believe he loves her very strongly. the truth is with guys like that, they can't really love anyone else until they fix themselves up (and he doesn't sound fixed up). he's going to "love" her because of the image she sends back to him. He feels safe in having her as his girlfriend, and powerful in asserting his muscles.
Yours is a tough position to be in. just keep telling her that no one in a healthy relationship should feel the need to abuse others like that. he's insecure and needs "the power rush", but he won't do her any good. and then (as others have said) it's up to her to get sick of it quickly.
just remember that when she does decide he's hurting her, she's going to keep wanting to go back to him, so your job will be to keep her busy and find her self-esteem again. oh yeah, and i'm guessing he won't be very happy with you for telling her to dump him. he might ask her to choose between you and him, or he might say you're trying to manipulate her, and that you wouldn't understand their relationship. just be ready for those kinds of things.