My friend eats a lot of unhealthy foods but still skinny?

Yeah I have this guy friend and he's been skinny since we've met. He's 15 now and through the years we spent together. He eats a lot! Well I was his companion in eating.. Sadly I gained a lot of weight through those years but now he's still skinny. He does not work out at all and is very lazy. He eats fatty foods and at night he can eat like 3 cups of rice. As I notice all that food did to him was for him to be taller. He's like 5'8 now. He also eats lots of sweets and carbs and still eats late at night. I can't follow him now since I became a vegetarian and lost weight.. Can anyone explain to me how is this possible that he's not gaining weight and just get taller?