Yes, Carnival charges $50 for a name change, and it must be done at least 72 hrs. prior to sailing. The new docs will have to be picked up at the pier since there won't be time for you to receive them via mail.
I would hope the person who backed out is gracious and reimburses you that $50!
If your DH hadn't 'agreed' to go, she would have been out the ENTIRE amount of the cruise fare....so she'll be getting off pretty cheaply by giving you the $50. Also, you would have had to pay more to occupy the cabin as a solo cruiser.