You are in middle school... All sorts of people go crazy in middle school, I was one of them. You can't do much, hormones are nothing you can control and if you step in it will do nothing.
I went from straight A student to straight F's in middle school, the only reason I passed is because I flew through all the proficiency tests. I started smoking and toking all in 7th grade and settled down after 10th grade. No one could stop me and if your friend is down that path why not just stay his friend and not talk about it. Don't be a downer, just don't support it, and hope he comes out of it. Don't get into the mix yourself though.