It's not a very complicated situation,

the decision was so obvious for anyone who's morally conscious.  Why need to be so calculating, need to know someone well? One side had a knife and was violent, the other side was non engaging with no weapons.   Most people i know would try to stop the violence regardless what's the cause and the relationship between the 2 parties. It's just the right thing to do. If people let it happen and do nothing, the next time it could happen to you.

BTW, i asked my son about what action he would take in this situation, he looked at me like "why this is a question? Of course, fight for or with my friend!