I am not sure a person with diabetes needs to do anything different than a person that doesn't have it to lose weight. Why would they? A healthy diet, smaller portions, a regular exercise regimen are all things that could help someone lose weight.
I think it is very important to tread carefully when dealing with teenage girls, body image, weight loss/gain, eating disorders. If this is something that she wants to do and you understand her reasons behind why she wants to do it, then there are ways to go about it. As I said above, those ways of going about it shouldn't have anything to do with having diabetes, given that she manages her diabetes in a sensible fashion (testing before and after exercise, etc).
Dad to Danielle, 15 years old, dx 8/17/2007, MDI (Novolog and Levemir)