My friend is slowly purposely killing herself?

I need advice, my best friend is slowly killing herself. She has suffered from minor depression (from what I've been told), for years. You see, when she was younger her father essentially abandoned her and now currently lives on the streets. Ever since then she's had problems and have been seeking treatment. That being said, against the judgment of her mom (who's always away on business) and her sister ( she looks up to, but is a bad influence), she has stop treatment, stating she's over it. Her father left when she was about 9, and she is now currently 17. She does not really openly discuss this with anyone, and I am pretty sure that she isn't wasting away her life to get attention. She seems empty. This year she has started drinking, doing weed and recently smoking. She has a very fragile body, and only two years ago went through heart surgery in which her lung collapsed. She physically can't handle this, and I can tell that she is doing it to waste away, her eyes are empty.