It may help the person realize that they are destroying a relationship with you if you can give them specific examples, like if they missed a prior commitment with you to get high. Let him know that using drugs is not something you want to be involved with.
This is really difficult to do because people ned to truly realize these things on their own before they decide they want to make a change. The best way to do it is let him/her know how you feel, maybe in the form of a letter so you make sure you have all the things you want to say.
Tell them and show them how it is affecting your relationship. Show them how things would be different if they weren't smoking pot. The best way is just to blunt with them.