An arm lock in a cramped space with a struggling kid can lead to all sorts of injury. I dont think the guy had the right to do anything, considering it really isnt anything to do with him.
It's actually good practise to do use something like an arm lock over say striking with a punch or a kick, it's why Jiu Jitsu is a fantastic martial art for Police and people who have to use restraints such as Mental health nurses. The arm lock the kid was in won't have been painful at all, he just won't have been able to move. Plus all we've seen so far is a picture, we don't know if the kid did anything else
I said I could have understood it if had been the case.