Nesseh said it well. I'd like to add something though. Your friend will grow up, we all do ...okay, well, most of us do. In the mean time, stick to your ideals but don't nag. Don't ride with him or her when he/she has been drinking no matter how much he/she minimizes his/her buzz. If he/she drops you as a friend, accept it, but keep him/her in mind to reestablish the friendship later when he/she grows up. Good luck!
Try a beat down. If that doesn't work, let them do what they want. You're not their keeper and if they don't listen to you, their friend, let them ruin their own life.