What happens to your body when you eat too much fast food? could you die?

i know that fast food is not good for you but the reason i ask is because i have a fourteen year-old cousin who eats nothing but fast food. i'm worried that he can get juvenile diabetes or even die. he has never eaten speghetti, salad, an apple, any kind of veggie or fruit, except for whatever they put on a cheesburger at micky d's. i am very worried and i was just looking for some answers, could my little cousin die?