Is there really an exact way you're supposed to stand at bat when playing baseball?

My nephew is good at baseball, but recently was told to not stand the way he had been, then after that he never hit any balls. I feel that the player should stand how ever they feel the most comfortable with. It's like that with anything, holding a pool stick, holding your pencil. Everyone's brains are wired different, so whatever one person does, doesn't mean it will work for another person. My brother in law got sooo mad at me because I thought he should go back to doing what he feels comfortable with. My dad grew up playing baseball and watches it too. And he didn't see anything wrong with the way he was standing before. He was standing with his feet apart, and one knee slightly bent, but they want him to stand strait legged with legs close together.