Wow. That's propper nasty. Im sure you trust your friend and everything. But is there anything at all he could have done to provoke this? Or anything?
Sadly, in England a lot of this happens for no reason at all, & it's increasing, let me give some examples, a 13 year old girl queueing in the school canteen was stabbed in the back twice with a metal styling comb, the puncture wounds were deep, needing hospital treatment, then there was a 16 year old girl a little on the plump side. who was held against a railing by several girls who pulled her school shirt etc down to her waist, exposing her upper body & calling the others over to have a look.
Boys are always being attacked like webmonkey's friend, if you look a little different like very thin, fat, wear glasses & for some strange reason, if they have ginger hair, anybody out of the norm provoke these attacks, when I was about 11, I was quite thin & about 5'9", & used to get pushed over, tripped up etc, 2 years later I filled out, putting on 20 lbs or so, so I went in search of those guys taught them a lesson they wouldn't forget.