Also, she's not teaching him not to share. She's teaching him that he has a right to say no if he wants to.

I work in a kindergarten, and there you have to share some resources. First, for equal opportunities, children are not allowed to bring toys from home unless it's a comfort blanket. If they do, they have to put them on a shelf out of reach or share with the children, as it's not fair for children with less resources to watch how the other children bring their super cool expensive toys and tease them. If a child is playing with all the kindergarten's wild animals, for example, s/he doesn't have to share until they finish, but if the child is having all the play dough that we set up to reach several children fine's motor skills, they will have to share some of it. It's important to teach children about sharing because they need to learn how to put themselves in other's shoes, but out of the nursery children should share their own toys only if they want to, and not due to pressure.

School is different, your also forced to be around people you don't want to be around. It's about learning not about etiquette. Those lessons are not all meant to be taken literally. They are taught to you on the premise that it will broaden your mind.

Also, learning to be assertive and say "no" even when it might seem difficult is a very important thing to know. I think teaching him this at his age is creating a good foundation for when he's an adult.

I agree! What you end up doing is creating exactly the opposite, the kind of greedy selfish person who doesn't want to share anything with anyone, ever. She's teaching him boundaries, and loving to share with close friends.