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.
45 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!