If your BFF was of the opposite sex, then I would recommend Aria as there is a coed outdoor patio pool that you can meet and they also have some indoor coed areas (heated marble tile lounge room and a room with lounge leather chairs that vibrate with spa music in the background)
If your BFF is of the same sex, then I would recommend QUA.
If you were staying at Venetian or Palazzo, then I would recommend Canyon Ranch Spa as it is only $20 for guests ($40 for non-guests)
If you plan on heading to QUA or Canyone Ranch, those are the only two spas in Vegas that accepts Spa Finder Gift Certificates. You can usually purchase $100 (2 x $50) Spa Finder cerficated at Costco for $79. The only catch is that they won't give you cash or credit back. If you use a certificate for $45 day pass at Qua, you forfeit the $5.
The Encore at Wynn is beautiful, but for $45, I would head over to QUA.