If God has stirred your heart to love your friend enough to talk with them about the sin they are stuck in and would like help in knowing how to do it well, consider looking at this, Confronting Sinful Behavior. This document has helped me a number of times speak with clarity helping someone see the seriousness of their situation and why they should repent. Also check out Caring Enough to Confront as well as Prepare Yourself Before You Confront to make sure your heart motivation is God-glorifying before you talk to your friend. I hope these will serve you well.