This is probably a good question to ask AA or one of the related groups. I'm not sure that you can do this for them.
They have to realize it themselves. All you can do is tell them your feelings. I was my mother that told me with tears in her eyes that I was going to die and she could watch me any more. http://www.al-anon.alateen.org/ is the place for you to look for answers
Tell them the way it's affecting the relationship, or career, education, or family members, or health. Then you walk away from the pain it causes you, and let them do it all in their own time.
there is a very tough saying around the tables. "unfortunately some of us must die" It's very sad but true
You say it once, they deny it, you tell them you realize they are in denial. That's it. Nothing more can be done.