Armed citizens have absolutely NO responsibility intervene. Your friend needs to mature considerably. He has no right to impose his personal views on the rest of us.
Point him to the news article this thread discusses and ask him if he's 100% certain that he couldn't make the same mistake playing "Batman".
http://www.thehighroad.org/showthread.php?t=659914
The guy I was debating this was a friend of a friend of a friend, hahaha. No one I'd associate with, especially after other remarks made in relation to this subject.
It turns out, the two men in question were involved in a road rage incident. The man with the knife was an "outlaw" biker, and the other guy cut him off in traffic. The biker caught up to him after he parked his car, and the fight ensued. The gentleman I was debating witnessed the incident, and claimed that he intervened without needing to draw his gun; I suspect he's a ninja master, personally.
The entire subject arose from an incident afriend of mine related to us; he was home, and heard gunshots coming from somewhere in the neighborhood. I asked him if he ducked and got away from the windows, and this other guy asked him why he didn't strap his gun on, and investigate, to keep the neighborhood safe. I replied that my friend had no duty to go and investigate anything; being an armed citizen, as others have already pointed out, doesn't obligate you to become a superhero, or a vigilante.
This began a debate on the responsibilities of armed citizens to their fellow man. Does being armed make you a sheepdog to the sheep? Does "self defense" only apply to self, or does it apply to society at large? I'm of the opinion that my gun is a means for me to get home safely to my family, and to protect them, as well as me. It's not for being a watchdog, or a hero. That's not to say I wouldn't intervene if I witnessed someone about to be raped or murdered, but I'm not going to go and investigate something suspicious, a la George Zimmerman.