Banerjek -- I think you did the right thing. I think we all have some responsibility as fellow travelers to help people out with mechanical problems. I always slow down and ask if people need help. Much of the time I am turned down, but on occasion I have been able to help out. I know when I have had a flat, I appreciate people asking -- I've not needed help, but its nice to know people will offer.
I think the guy you ran across was being stupid -- not a condition any of us can fix briefly at roadside. The guy made a poor choice to ride unprepared after dark and that is not your responsibility. Think of him in the same camp as the wrong way cyclist you see. You're not responsible for the wrongway rider or riders striving for invisibility.