@cornelljdav,
Then if it happens again, tell her to call the cops, or go to a hospital.
Her telling you, after the fact, does no one any good. What does she expect? That you'll drop everything, go running over there like her white knight and beat up that guy?
It is a criminal matter and likely also a health matter. You are an expert in neither.
But do Google
battered woman syndrome.
http://en.wikipedia.org/wiki/Battered_person_syndrome
This behavior is kinda typical - and it can take a while to convince someone to leave. But you can be a friend, and continually tell her that she can do better. Often abuse victims suffer from low self-esteem. This is not a time for you to be trying to date her - this is a time to be shoring up her self-esteem, telling her that she doesn't have to live that way.