When he was arrested, did the Police ask if she was prepared to have him home? If she says no, then the Police have to contact Social Services re accommodation. However, Social Services will do their upmost to put pressure on for her to take him back. The sad thing is, as it is him assaulting her, SS are often reluctant to get involved rather than in the cases where it is the other way round.
Was he charged? If so, Youth Offending Team should have been informed, and they are the agency that can provide parenting support and intervention etc. Depending on the area your friend is in, and how stretched the agencies are, things have to get really bad before people get help. I know that they are already shocking, however, sadly, there are so many cases like this these days, almost like its starting to become the norm, as crazy as that sounds.
Boot camps cost a fortune, but do work for some, but often only for the familys that can afford to keep their kids at camp for months on end until they are so desperate to come home they will learn to behave!
Womens aid would be an excellent place to get advice too, especially if your friend doesnt want to go down the care route, and as I said, even if she did, Social Services are governed by purse strings, and will only re home in the worst case scenario. I used to work in a challenging behaviour unit, and we cost �4500 per child per WEEK!
I hope that things work out, I have experienced very similar situations in my family life and through work, and it is extremely difficult for all involved.