Thanks everyone for the replies. As I expected, it's a mix of encouragement to try to get her help, and some to leave her be.
Given where I am at with her, and my own experience with addiction (I'm a recovering alcoholic) I feel that I need to let her parents know what's going on. I know she'll be angry with me in the short-term, but once through a program I hope she understands that I'm doing this because I care for her. She deserves better.
Just a month ago she was talking about what she needed to do to get cleaned up and what she would do after. Most recently, I've seen some serious changes in personality, which makes me think that her habit has gotten worse.
During one of our conversations, she told me that he father was an alcoholic that had been sober for 16 years. Although I've never met her father, I do know how to contact him and was thinking that he may be the most understanding and capable of handling this due to his own experience with addiction.