Well yes, for grown-ups (parents, teachers,...) it is understood that they know the technique of "criticize the behaviour, not the person". In other words, that they would say "wow those are some ugly things to say" rather than "you have an ugly heart".
However I would not expect a young boy for not knowing or using this technique, and I think his bluntness was to be preferred over just taking whatever she was dishing out to him. I don't feel sorry for the girl (well, I feel sorry for her that she feels the need to that behaviour, and I do hope her life/attitude/whatever gets better, but I don't feel sorry for the backlash she got from your son). In short, if she can't take it, she shouldn't dish it.
Since you /asked/, I think it's ok to mention to him that "address the behaviour" technique, for possible use in the future, but again, I definitely don't blame him for being as harsh as he was.