It's this childish "don't tell" menality that is part of the problem with today's society. Problem aren't solved by trying to get the upper hand on someone else and act like the tougher guy. This is why crime, violence and gang activity are so rampant in the city, and becoming so popular with the youth. It's showing defiance in a time in their life when they should be learning more about the world. Instead, they've built a world around themselves and threaten anyone who's on the outside. If you're not "in", then you better be. Don't cross us or face our wrath.
It's stupid (a poor circumstance) and arrogent (the person who did this) that this sort of thing happened. If you know who did it, get the cops involved. There are ways of approaching every situation without violence. Having a neutral party contact the cops without giving contact information might help.
Did your friend even do anything to provoke the attack?