oh, i understand what you mean, i just stress the difference between the logic of a situation and the feeling.
is it unreasonable to expect your child to be invited / have a go at the parent if your child isn't invited / not invite child in retaliation - yes
is it unreasonable to feel upset (on behalf of your child) - no
your feelings are your feelings, they aren't unreasonable. your actions - now that's where you get into reasonable or not.
i don't know about other mums, but i feel very emotional about my boys, and very protective of them and i feel rejected on their behalf. would i make demands on other parents - no.
do you get me?
so my opinion on the op question is still no, she isn't unreasonable to feel upset. she would be unreasonable to go up to the mum and demand her child be invited, but that's not what she asked.