Absolutely terrible diet. It has already shaved years of health away from her she just doesn't know it yet.
Chicken nuggets aren't that bad for you but you can't eat nothing but fried meat and no vegetables. High protein diets make you age faster.
Is she also drinking soda and french fries? Cut back on the portions of the nuggets, replace the the fries with some salad (with a spare-to-moderate helping of dressing), replace the soda with unsweet tea, add some beef, and get enough exercise and it isn't a terrible diet. Trying to go full abstinence on the nuggets will probably fail because her stupid mother's years of neglect. Of course, she should learn now how to save money by doing it at home.
