OMG grinning what a horrible night you've had. From memory someone can be sectioned by a close relative (your friend) if she has the backing of 2 doctors - maybe his GP and his therapist. I'm not sure how it all works - but her really sounds like he needs help.
I did a lot of work with the police when I dealty with domestic abuse cases - sadly many of them don't really know the law or what their powers are. They are very quick to say "it is just a domestic" - but your friend has been assaulted so they should take it further. Sadly this type of man is very good at appearing like a normal bloke to those in authority.
I know in Scotland this sort of thing would be reported to the Procuratoir Fiscal who woudl decide whetyher to take it further - but I think in England more regard is taken of what the victim wants to do.
There are lots of protective remedies your friend can get to keep him away and have him automatically arrested if he breaks the court order.
The best place for advice is Women's Aid - they are very non-judgemental and will really help her (or you if you phone to get the knowledge to help your friend)
This sort of situation really breaks my heart - and it is so often the case that the victim stays with the abuser or keeps going back - it is such a viscious circle. As you said at leat there are no lo's involved (I have represented women with really harrowing tales where there are kids involved too).
Well done on keeping Amy close to you - you and your dh needs to break off all contact with this man as much as you are able to while supporting your firned.