Re: Some what of an unusual situation?
The trouble is, in my experience, that most people won't stop using drugs until they get to the place that they want to stop. Does that mean you do nothing? No, but you might feel like you're banging your head against a wall in the meantime.
Where she is your ex, you're very limited as to how much influence you carry with anyone close to her. You're likely to come across, I'm sorry to say, as the jilted, ex-lover.
By the time a person gets to be the age where they're using drugs, their parents don't have much pull anymore. Unless the girl is a minor and they can force her into rehab (~if~ they believe you). You might need more evidence than your word. There are really no good answers here, but you are certainly right to be concerned, and I'm not saying give up. Just don't get arrested for harassing her!