I work security for a company that provides for large events and clubs. I get to see lots of fights. But I have never seen 5 on 1. I have seen 2 on 1 with three encouraging the brawl. But not 5 activly distroying one person. The last time I had a unfair fight on my hands I grabbed the aggressor and threw him off of the "bottom" man. I had my security shirt, vest, gloves, and walkie. I was using my best DEEP voice and yelling at the top of my lungs to stop. The top guy I put in a rear naked choke and spun around to keep tabs on his pals.
I knew that the other guys I work with would be there in a few seconds and im the smallest one(6' 220). My confidence is in the fact that I know I have a few monsters watching my back...LOL. Out on the street they would have to be killing the guy for me to step in. If its just a little punching, no biggie. Trying to kill and yes, I would help.