I have a good friend who is doing drugs and I wish they'd stop
It can be very hard to watch someone you care about make bad choices. But it is also difficult to tell someone what’s best for them. People mature at different ages, and sometimes you are ready for what’s next in life, but your friends are not.
Don’t tell them what to do, but do tell them how you will be.
“I won’t be around you while you are doing drugs” is a good thing to say. You can still like the person but not like some of the choices they are making. Separate the two. It’s possible that your friend might want to stop, or they might not be there yet.
It’s OK to like the person but not the habit.
But don’t put yourself at risk to “prove” your friendship. What kind of friend would ask you to?