My friend's son started preschool this January, n his second day at school, he hit over another child with a pillow. How can his dad really make him understand that this is really wrong. Even if he is punished he will not be able to associate the punishment with the wrong he did, since they happened at both times.

He can't really. He's going to have to hope the teacher made it clear that hitting even with a pillow) isn't acceptable. If it happens at home he will have the chance to teach his child himself, but for now he will just have to wait and see.

I guess for every parent, they hope that their little angels will remain as little angels instead of growing up and getting into all kinds of trouble.