You're the strong guy now in the relationship. Strong in the sense that you're not into drinking. If you think you will be tempted to follow his drinking habit, then you have to end the friendship because both of you will just sink each other down. But if you know how to resist him and can in fact, help him stop it before it's too late, then do what you can do. It may be easier said than done but you have to be really determined and resolved at not having a go at it again. Good luck to you!