I usually fight to keep my mouth shut unless I see someone about to do something really, really foolish. Then I might say "You know, you'll want to keep your brake hand on the rope when belaying" or "You probably already know this, but your 30m rope's not going to be long enough for that route." Or whatever. I don't want to be one of those people who's always offering unsolicited advice, but sometimes you've just got to say something.