It is important to understand what is driving them to consider going down this path. Perhaps you can help them deal with the underlying issues. Talking with them is essential. Try to get them to discuss what’s driving them in this direction with their parents or some other adult who can help them.
But once (if) they decide to actually go down that road, your friendship will have to change. Not end, but change. You should no longer actively keep their company, especially if doing so brings you into an environment where other drug users will be around. Your friendship will have to take a more distant form, which is just as valuable. Your friendship will have to be one based on prayer.
This may seem like you’re abandoning your friend, but you’re not. You can do more for them by praying for them and offering up the pain of your separation than you can by following them into a group of “friends” who will lead him or her (and perhaps you) into a life of drugs.
My sister recently gave my 15-year-old daughter the book “Go Ask Alice”. I read it, too, and it is a real eye-opener if you’ve never seen it happen before. If you’re a teenager, I recommend asking your parents if you can read the book. It is a diary kept by a young 15 year girl who had a lot going for her, but felt very out of place because of normal adolescent feelings and some turmoil in her family. She was led into a life of drugs and sex by a “friend”, who clearly was no friend at all. It is a terribly sad story, but I’ve seen it happen so many times it’s hard to count.
I caution you very strongly against maintaining an active friendship with someone who has chosen to abandon true friendship with you by using drugs. You can still be their friend, but your prayers and sacrifices, your example of staying clean, and perhaps even the realization that you won’t spend time with them anymore, will do more to bring them around to the light than following them down that road.
Remember that God is their best friend, not you. He is able to help your friend far more than you can. If your friend chooses to use drugs, then have faith that God will use other instruments to help him/her, instruments that are better equipped than you to save him/her. His/her parents, or other adult family members. Counselors or teachers at school. Doctors. It really is out of your hands once they take that step. But they’re still God’s loving hands, and he will not abandon them.