Simon hears request, asks Rayna to play:

Rayna was in the audience calling for Simon to play the song “Duncan”, adding that it was the first song she learned to play. Apparently the songwriter heard her and motioned her on stage (much to her astonishment). After a slight case of nerves, she took the guitar and began to sing and play the tune.

I think she did quite well considering she’d had no rehearsal and was put quite literally on the spot in front of a crowd and a musical icon.