Sure, maybe the cool kids dont bring their lunches, but why not question that socially constructed idea? ie) You know what is cooler than not bringing a lunch to school? Bringing healthy lunches, and protecting your body from damage is way cooler than eating chicken fingers and poutine from the cafeteria.
Moreover, what you may not have realized is that in general, there is a outcry to society to drop your video game controllers, turn off the TV and get active and healthy. What better way to get healthy than follow a strict gluten-free diet which forces you to maintain a healthy standard in your meals?
It is COOL to be healthy, and I know a lot of my friends used to rub it in my face that I wasnt as healthy as them.
Try and boost her mood by enforcing the idea of healthy living, and make sure she has an edge over those "cool" kids simply because she is going to be feeling better and healthier than them in general.
Sorry if that seemed sort of choppy, but hopefully you find it useful.