Why is Mangia "he eats" and Lei Mangia is "she eats." Why isn't it Lui Mangia?
It's a context thing. So if you were talking to someone and the conversation was about a boy, you could say Mangia and the other person would understand what you mean. Therefore, the sentence isn't incorrect as much as it might seem kind of odd without context behind it. It's not like in English where you can't say "eats" and have it be grammatically correct, because you need the "he", "she", "they" part of it to make it a correct sentence. In Italian you don't need that so there are certain sentences that don't make much sense unless you know the context.