We were allowed a lot of freedom as kids, mainly because we were responsible and never got in bother. We knew full well we'd be crucified if we ever got in trouble. so really appreciated our gift of trust - I guess you have to let her prove she can be trusted some time, so it's time to cut the apron strings.
I think it shows maturity that she's asked you and was honest about what she's doing, rather than pulling the old "we're all staying at x's house" and sneaking out camping.
I say let her go, and as much as it pains you, leave her be for the night