.. and there was dancing involved (huh?)
The singer joining U2 on stage is Francisca Valenzuela. Look how B (at 1:55 in) points out the teleprompter to her. Look! Karaoke style One Tree Hill, babe!! Odd. Would be cool if (just) the band played this one again.
(Thanks to @U2gigs for capturing it in quality video.) The rest of the U2 setlist from Santiago on March 25 can be found here. Muse opened for U2 (Muse videos and setlist from Santiago here).