what would you do if a guy hit a girl you know while you wasn't around?

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.

You are a roomate and a friend. You are not her bodyguard.

Give her the information to safeguard herself if this should every START to happen again:
1. only date guys that don't hit women.
2. police phone number is XXXXX

Don't let her drag you into this DRAMA. They will BOTH turn on you if you get involved, believe me.

good point

that's right. I ain't no body guard, I'm just a friend.

I wouldn't wait til then.

I'd make sure that she understands she needs to call the police if anyone assaults her.

that's right.

Get the address and call the police yourself.