My first thought....have her thyroid checked. If it's not that, I'd visit your health care provider. How much weight are we talking?
As far as July 4th...I find summer holidays to be the easiest when it comes to parties. You can easily grill just about any meat. If you want to keep it simple and not worry about marinades or seasoning just do a burger or compliant sausage or hot dog. Potato salad and broccoli salad can easily be made with compliant ingredients. Fresh raw veggies with a compliant dip or guacamole and fresh fruit (watermelon slices!) are good options too. If you want to get creative, you could try ribs with a sugar-free rub and sugar-free bbq sauce.
For lunches these are my favorites (I'm not a teen, but I have 2 of them and they eat all of these):
Salmon cakes. http://meljoulwan.com/2014/11/10/oven-fried-salmon-cakes/ They freeze beautifully so I always make a double batch. Extra special good with Awesome sauce (recipe is at bottom of page here: http://meljoulwan.com/2014/03/10/bacon-jalapeno-burger-balls/)
Tuna salad, chicken salad or egg salad are all good lunch options with a side salad or raw veggies.
Deviled eggs with prosciutto or compliant deli meat with a side salad or raw veggies.
Taco salad with ground meat, onion, seasonings served over lettuce with pico de gallo, avocado or guac and compliant dressing (Tessemae's chipotle ranch is my fav)