Depends who the friend is. I only have a close circle of friends I actually "care" about - others are just pub friends, etc.
With the 10 I care about I can quite simply tell them they're doing this wrong and try to explain why it is wrong. If they decide against it, then it's their option, although I'm not going to support it and I'll probably bring it up many times whilst drunk. If they choose to take my advice (which rarely happens!) then I find they're more thankful then annoyed. Possibly because the conclusion was better than the dilemma they were in.