I hate to say this, but you're probably going to experience this a lot as you and your friends grow up and start making their own (often idiotic) decisions. It's so painful to see someone doing something you know they're going to regret, and also realize that any attempt to help them is only going to drive them away.
I think it happens to women more then men. My wife has lost a lot of friendships (some with family members) over bad boyfriends. Inevitably they wind up breaking up, months or years later, and then saying "I don't know what I was thinking. We should get together again sometime." But it rarely happens.
With men, the pattern seems to revolve around drugs and/or alcohol. You just can't talk to an otherwise smart person who is constantly high and hope to make a difference. All you can do is hope it runs its course and they come out the other end intact.
That sucks :/ But that's what annoys me the most, you know? Because I was always aware that if I pushed too hard on the subject, she would go away, she would close up and not talk to me, and I always tried to be gentle, to keep her talking to me and still, here we are.
It's annoying.
It's sad to lose friends over such silly things :/