Mana I'm not sure about what state your in but when I was looking I saw no state that makes it illegal to leave a 16 year old overnight. (14-15 seems a more normal cutoff) But that doesn't mean you don't have local rules. I could imagine leaving my 15 year old one night but not much longer.
Actually, I took their word for it and didn't bother to look it up even though I thought it was really odd. Anyway, I just googled and our state has NO laws on this whatsoever. Now I am wondering if they themselves were misinformed or if I misunderstood them but it'd be too awkward to ask now.
You learn something new everyday.
