Merlin - I think it's very likely to help.
Actually, I think the stairs exercise would help everybody , as what it does is improve the cerebellum.
A carb heavy diet is a symptom of cerebellum problems.
You can start by either going to Tinsley House, or buying the Brain Food Plan and following it. The stairs exercise is explained in there.
Given that she's 16 you need to help her as quickly as you can, therefore if you can afford TH (£250 for first appointment) I would highly recommend it.
If you can't afford it, then keep talking on here, and we'll all help you as much as we can. But obviously it's better to actually see him.
Particularly since you're the only one with a teen, so maybe his advice to you will differ slightly. I don't know.