Not in the parenting business
First, I'm a parent (three times) and you can bet I watched where my kids went and what they did even (and especially) during their teens. That being said, I have seen parents drop off 12 year olds at an all night rave without even inquiring about what went on inside! I usually recommended they go inside and check out the drug-induced sex going on upstairs, which usually resulted in kids being taken home but the large majority stayed.
I don't recommend kids under 12, but I am not parenting somebody else's kids. If they want to let their kids go through its their decision ( poor kid). Usually the kids end up having a great time though watching Dad piss himself and scream like a little girl!