M.O.
If you left clear instructions on when you expected him into bed and she didn't follow those instructions she'd be DONE in my book.
Unfortunately I think this is an all too common issue with using "teenagers". The reason they aren't considered adults yet is because they lack some of the judgement and reasoning.
Think about it, not only is she responsible for your son's care and safety, but your home and everything else in it. What if she decided to "have some friends over" after your son went to sleep? Or have her boyfriend over?
The flags have been raised, do yourself and your son a favor and don't use her anymore. Maybe it's just a "bad judgement" incident, however this is your SON we're talking about, not the family dog.
I'd take immediate steps to get a new person to care for your son.