I would say that you absolutely SHOULD tell the parents.
But then I don't know if it would do any good. Afterall, it seems to me, that if the parents cared, they would already know what the kid is doing because they would be right there, watching.
Your momma know you are here? :')
I think many parents make the absolutely rational decision that it's a mistake to completely shelter their children right up until the age of 18, as you seem to think is appropriate. That doesn't mean they "don't care" as you suggest.