I would ask a _competent_ Rav whether to inform the parents.
On one hand, parents have a reasonable approach and pursue the best for their kid.
On the other hand, they are failing and it is not clear that doubling-down will help. For example, if kids break through filters on a computer and parents discover it: if they close the break or confront the kid, he will find another way and parents will never know about it. It may be a better idea to monitor the break and see whether this is benign or not. So, in this case, maybe a Rav and friends can help the kid using some other approach instead of encouraging parents to follow the path that is failing. For example, maybe the kid has questions that need to be answered, or he needs more warmth and understanding.