I think the instinct to be helpful to someone in trouble is always wonderful. I've encouraged it in my children, in fact, I've pretty much taught them that it's reprehensible to look away.
But I've also taught my kids that they don't necessarily have to confront the bully. Sometimes, in fact, many times it's enough to just walk up to the potential victim in a situation and take his or her hand - providing a distraction without being too confrontational.
A similar situation unfolded in front of me about a month ago in the middle of the city street in Oxford. A friend and I were walking down the street and we happened upon this very effiminate looking young man of about twenty who was involved in some sort of altercation with an older, huskier and rougher looking guy. As we watched the bigger guy hit the younger guy in the face. The young guy grabbed his face and stood there crying. The older guy was yelling something at him - I don't know what. My friend kept walking, but I turned around and said to the young guy, 'Come here, come walk with us,' because the big guy was advancing toward him again.
I walked back over to the young guy and took him by the arm and said again, 'Just walk with us - he won't bother you then.'
He walked with us around the corner and then went on his way.
My friend was a little pissed off at me - he said, 'Yeah - you go to save the day, but if that guy'd wanted to fight someone, he'd have hit me, not you.'
I said, 'He didn't hit either one of us - and he stopped hitting the guy he was hitting.'
I would never advise my children to walk away without at least calling someone else for help - but again - sometimes just showing the bully that other people care for the person s/he's bullying is enough of a distraction.
Of course if someone has a knife or gun - I'd have to rethink the whole thing.