Sometimes, hard as it may be, you gotta let someone go. If that person doesn't want to change, no amount of caring and supportiveness is going to make them change.
My sister, is going through her second bout with cancer right now, and she still won't take care of herself. She's used drugs since she was about 14--she's 39 now--and has no intention of quitting. The first time she had cancer, she would go to chemo every other Monday, and so she'd have one week where she would feel bad, and one where she felt good. And let me tell you, on the off weeks, she'd go out drinking and "partying" like there was no tomorrow. And to make matters worse, she won't hardly eat. And even though the Dr told her that smoking cigarettes would lessen the effectiveness of the chemo, she continues to this day to smoke. I kinda think that's why the cancer came back so soon like it did--she was in remission in October, and it came back in March. The past few weeks she's been in the hospital for a bone marrow procedure, and Mom caught her sneaking out for cigarettes. One time she ran away from the hospital, and they had to call my other sister to help them locate her...
And it's hard, but What can you do? She's an adult, and she obviously doesn't want the help.