You can only control your own behavior.
And, it sounds as though you may need to tell your friend that you cannot be there when they are high. Period. If the friend wants to do something with you, or hang out, they must be straight.
You may not see your friend for a while, or often; but it is important for people to realize they can and will lose precious things and people in their lives if they are self-destructive.
Speaking of which-------talk to someone about how/why someone else doing something to hurt themselves triggers your own desire to self harm. That is significant.