This is a toughy.
As a parent, I would say I'd want to know. (Assuming we're talking about an underaged person, because they can't really get help without their parent's knowing.)
Even if it was just an anonymous letter mailed to me with suspicions and no real proof only concern. Then at least you told the people who can actually help. They can look into it.
I'm not saying that's the BEST plan, just what I would like. I'd assume the person in question has heard it's dangerous, and don't care.
That is the biggest problem with being a teen. Parents are always out of the loop and they can help, their friends can't help, but they know.
IF this were to end in something life threatening, parents knowing is better than anyone else.
Also, if your friend hasn't confirmed it with you yet, you're TECHNICALLY not breaking a confidence. Although, I wouldn't tell your friend that because I doubt they'd see it. That would just be for your own knowledge.
That's a tough call though. I can't really advise on WHAT to do, just what I would want as a parent. As a teen, that's the LAST thing I would want you to do.
Thus, I'm very unhelpful.