Bait it with tuna and wait. I would strongly reccomend keeping it in a crate till it's owner returns. Cats are very sneaky animals.
I disagree that it's necessary to keep the cat cooped up from here on out. Let her hide, she'll probably become really comfortable and accustomed to the place about the time the fiancee comes back... If not, you now know what works to get her to come out. Close off rooms you don't want her to be in perhaps, and just relax :~)