If you knew someone else's DC had done something that you know the parent would not be happy about, would you tell? Nothing criminal, nasty or dangerous as such, but definitely outwith the parents usual boundaries/comfort zone. My own DC was involved and isn't in trouble as such, but knows it must not happen again. I don't want to stick them in, but the other way around, I'd want to know and be able to tell my DC it's unacceptable in future.
Sorry to be vague 