What should I do?

I need another Ethics class...or at least a lesson real quick. I don't work there anymore, but I have evidence that one of my girls (I was her primary counselor) is smoking weed. Am I obligated to notify them, ethically? Or is it ethically WRONG to notify them (since I don't work there anymore and I have no business there...HIPPA laws is what I'm thinking)? I spoke with my boss (ex-boss) today and she has me as relief staff right now (per diem, as needed), as of 3/21/09. (I JUST found out about the "evidence") So, I guess that still makes me an employee and I can notify someone? This is for the best interest of the resident. I was her Primary Counselor...I even did her complete intake. I feel like I have a responsibility to notify someone there, but I don't know if it's ethically wrong to do so??? My purpose isn't to start trouble or get her in trouble. My purpose is to keep her safe. I guess that's one of the things we were told to look at when making an ethical decision when I took this class in college. But, technically, I DON'T work there anymore...I don't know what to do!!! Does it go against HIPPA laws?