Basically, she starts out in a soothing, higher-pitched female singing voice.  Then, halfway through the song, HE switches to a deeper dude's voice . . . and continues with the song.  Either way, the crowd and the judges loved it. 
(--The switch happens at 1:04.  You can hear 'her' speaking voice at 4:26.  It's deeper, like the second part of the song, but could definitely still pass for a chick.)