Do they have to be frozen? As long as your sister keeps them cold, almost any fillings would work.
How about buritos filled with rice, beans, cheese, meat or chicken, veggies. GB posted this before and I tried it and loved it. I used chicken, cooked onions, rice, refried beans, red pepper flakes. I rolled them up then placed them in a baking pan, topped with salsa and cheese and baked. Your sister could skip the last step and just serve them up sliced pin wheel style and bring a jar of salsa to have on the side.
I've also made smoked ham, honey ham, turkey, and cheese wraps, with horseradish sauce and dijon mustard.
If she is doing a camp fire, she could use a cast iron skillet, throw a wrap in to brown lightly, removed, then add veggies of choice ( I used red and green bell pepper, red onion, mushrooms, sauteed them then added hot salsa, and cheese. Add this mixture to the cooked wrap, then cook another wrap and place it on top. Cut into wedges and serve.