To intervene or not to intervene?

Heading home from work yesterday I hit a moment of conscience when a loudly fighting guy and girl now designated A and B burst out of the tiny deli near my local train station and did a sort of angry yo-yo thing in and around the corner. I usually hang about if something like that flares up near me (presumed couples fighting) on the theory that at the very least having a witness doesn't hurt.

A dude further down the road and I hung about and observed for a little then wandered back to find the deli guy (F) with overturned chairs. The crowd of shouting people expanded to A & B plus 3 more guys (C, D & E) and included such winning phrases as ‘I’m not afraid to hit girls!” (C) My brother is going to get you!” (B) “He had a knife!” (A) the always awesome “I had to defend myself!” (F) and my personal favourite: “this is why we left Africa!” (D?)

I have no idea who started what; it vaguely seemed to be about someone (B? C?) wanting money from someone and B telling A off a lot but mostly it seemed shouty. We hung about and said soothing things like “Are you OK?” “I’m uncomfortable with all the threats of physical violence here” and “Would you like me to call the police?’ until eventually it wound down and the shouty people wandered off and caught a train. One of the guys (D?) did some sort of complicated but obviously peace-making handshaking ritual with Deli guy who was tiny and later said A & B had come in and A had tried to ‘fight him.’

I’m not sure what a really skillful intervention would look like, mostly I just provided uninvolved calmness - any suggestions on how to handle situations like this in the future?