DH often people who are hit make sure they stay covered up, as they are ashamed, do not want people to see etc etc.
Yeah it would be good for her to speak to the police, counsellors etc, but a genuinely good alternative would be to get the **** kicked out of him. I say this in all honesty. The police often just give lip service to crimes like this, and the victim suffers the fallout. A serious kicking would make him recognise the severity of the situation and the repercussions that could occur should his behaviour continue. Yes this is being just as bad as them, but if violence is the language they speak, then they should understand it.
As she is 'choosing' to stay in this situation (through fear, misplaced loyalty, a belief in 'he's not really a bad guy, he's just had a hard time' or whatever), she cannot be expected to get herself out of it. Whether it's through taking her to her auntie on the other side of the country's house for the summer or whatever, or just sitting her down with her parents, or a couple of trusted friends, she needs to understand the situation she's in.
And while she's doing that, send round the lads with the 'physical warning'. Trust me, this works.