The other day I was hanging out with a friend who's a heavy drinker. I was hoping that by accepting his offers of shots that we'd kill the bottle and he'd end up drinking less, but he just went and bought another. But I don't think he would have drunk less without me there.

If they're only doing drugs as long as they have someone to do it with, then the answer is maybe. But chances are that's not the case, and you are not enabling them. Sometimes expressing how much you care about them, encouraging them lightly to have moderation, and keeping an eye out for them is the most you can do.