eye for an eye leaves everybody blind....
not that i actually beleive that, but in this situation, taking justice in your own hands would only make things worse... if he's done that just becuase he was drunk, what do you think he'd do once he found out she went and told complete strangers what happened, and he suffered a beating becuase of it? she'd probably end up being beaten like 5x worse...
and taking her to the police? ok i'll tell you what would happen... they'd see the evidence of what happened, and in most states whether she wants to press charges or not, he will go to jail... for that night and MAYBE just MAYBE get a restraining order.. which is also useless unless she would call the cops if he violated it, or beat her again in which the police got involved yet again (her being volunary or not), then he might get a court date, be up for a max of 10k bail and be right back out...
as sad as it is, sometimes you do have to think ahead and realize the justice system doesn't always work... it works fewer times then it does work actually... so you have to think about her, and what she will be facing once he's once again free...
so i'd say you made the right decision... perhaps you could talk to her more and convince her of what can happen if she doens't do something soon... but otehr then that, it's out fo your control....