My friends boyfriend hit her, what should I do?

My two best friends started going out when they were 13. They're 17 now, and just recently he's started being violent. I know them both really well, and can't imagine him being violent. He's always been (at least in front of me) very calm and caring towards her. She can be irrational and get mood swings, and because of this they often argue. Four months ago he pinned her down twice and was a bit rough during two heated argument. She dumped him and they were apart for three months. Recently, they got back together. He was incredibly sorry for what he'd done and had even called me crying about it and asking what he should do (I told him the damage was done and he should leave her alone). Anyway, they got back together, and yesterday they left to go on holiday together. But she called me to today in tears because he properly hit her today, and there's no way for her to leave without him so she's stuck there for a week. She doesn't want to dump him because she "loves him too much". I don't know what to do about it or what to advice her, but I'm really scared about it and can't believe the guy I've known and trusted and been friends with so long could turn on her like this. She still has faint marks on her arms from the first time he got violent months ago, and she says he's left a much worse mark this time.

What can I do?