Depends on what kind of sailing she does, really, but if she's a "normal" girl in a windsurfing family but not a hotshot working on tricks and moves, AND if weight is an issue (my experience for youth up to early-/mid-adolescence), then I'd probably suggest a KS3 or KS SPIDER. If overall durability is an issue, probably a Super Freak classic.
Freestyle sails are made to be really powerful to pull up to a moderate speed plane for freestyle aerials. So they tend to have a lot of draft and tight leeches good for pumping the rig. I haven't used the Freestyle, but reading others reports and what I know about freestyle and gear, as a purchaser this is what I'd be looking for in a freestyle sail of any brand. If she's into that, or maybe even close to that, it might be a great choice.
But if she's a "normal" mid-teens girl in a sailing/windsurfing family who is working on the usual progressor type of things (harness, jibing, wants to waterstart because she gets tired uphauling, etc.), then I'd go for something light and forgiving in gusts. In that case, the KS3 or Spider would be the call. A less powerful choice (i.e., not pulling as hard) would be the Super Freak, which I find to be pretty light but is heavier than the KS's...but if she wants something bright, colorful, and durable as heck then a Freak would be a good call.
Hope this helps...