I don't know that I'd call that beating "brutal." By his own account, the victim had a scratch on his nose. No broken bones, no apparent ER trip. The perpetrators looked more like they were trying to embarrass him and impress each other, both of which seemed to happen. In this case, I would have taken diligent mental notes for the police and would have offered aid afterwards, but would not have risked escalating the fight and causing this guy/myself/others potential for greater bodily harm.
Now, if we're talking about grandma in the same position....
One has to make these assessments. Decisions have to be made.
Make the wrong one and it could alter your life.