My friend thinks because he's thin and active (plays soccer, weight lifts) he can eat junk food?

Not true: He may have an active metabolism and not gain weight by eating junk food but, like anyone else, his overall health depends on solid nutrition. Eating excess sugar, fat, and salt can prevent him from getting the proteins, "healthy" fats, vitamins, and minerals his body needs for proper growth, repair, and prevention of disease (such as infections, cardiovascular disease, diabetes, and cancer).