I can't address the visa questions, but I can answer the cabin question.
The Celebrity Equinox doesn't have any single occupancy cabins. They are all, as a minimum, double occupancy. You can book a cabin as a single passenger but you will pay what is called the 'single supplement' meaning you essentially pay for two people, but the port fees and taxes are only charged for one person. I often go on cruises as a solo passenger in a standard double occupancy cabin. It happens on every cruise.
Her manager may not be aware of the cabin categories, or perhaps something was lost in the translation and he meant that you would have to do as I said above.