You can do it either way; if you tell them now, they'll refund the taxes for the second person now. If you wait until you check in at the pier, they'll refund your taxes then.
Sucks you can't find someone else to go with you because, as ruth stated, they would only have to pay the taxes (and of course gratuities).
Perhaps you might want to look for a roommate - plenty on here are looking.
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!