i think that parents who know their children are rarely surprised by their behaviour - very few children are angels and have never started anything, but for most, this sorts itself out and they work out for themselves how to behave (with guidance from their parents). However, you always get these types that refuses to believe that their darling could ever get up to anything. invariably their kids are the worst because they have never had their behaviour checked by their parent. The school will be well aware of those types of children and those types of parents. Your "friends" child is what we in Scotland would describe as sleekit. It sounds like a terminal case - Dot and Nick Cotton spring to mind!
Personally I would leave her to it - doesn't sound like much of a friend to go behind your back to the school rather than speaking to you privately