As someone who works with teenagers, there is always a fine line to be toed: do I cave into societal demand and dress the girl like she's a teenager going on forty, or do I dress her like I'm about to send her like a nunnery?
Unfortunately, I've found that most of the time my clients end up going with the former. I have an ex-client who got a facelift at twenty. I recommend Betsey Johnson, Vivienne Westwood, Prada, or anything from Banana Republic.
You could, however, also take her to a tailor and have something bespoke made for her. That's expensive, but would last longer. I wish you well, friend.