My good friend has become a huge pothead, and I want him to stop. What's your advice?

My friend used to be really funny, smart, outgoing, and just fun to be around. He said to me personally a few months ago that he'd only smoke pot on special occasions, like only a few times every few months. Now he does it at least once a week. He's making all the bad decisions, acting like a total douche sometimes, and his intelligence is really being put to waste. My other friend has a pot pipe that he brings to school sometimes, hiding it in a stuffed animal and smoking it after school, and my friend smokes with him a lot too. Now I learned that this guy's been talking crap about me behind my back, and he's been acting like a douche to me lately. Should I tip off the school administration about the pipe so they can get rid of it, and maybe my friend's problem?