I would have yelled...
I understand that I shouldn't for all of the reasons given, but I always say something to unhelmeted riders. Once, I said something to my neighbor, who was riding down the wrong side of the road, unhelmeted, with his kids.
And frankly, I would like to say something to people engaging in other unhealthy habits (like stuffing french fries into their mouths when they are 100 lbs. overweight). I know I shouldn't be policing other people's bad habits, but I sure feel like it!