Agreed, No one should get a medical procedure just to take a cruise."
But this child is unvaccinated and could cause issues with the family who will be her chaperones. As such, they have every right to 'go back on their word' IMO. It is their vacation that she has been asked to join. If the fact that she is unvaccinated by her choice or her parents is going to cause issues on the cruise, then personally I would tell her and her mother, that she is no longer welcome due to that fact.