She couldn't attend the concert because of her failing health, so they brought the show to her. 

Florence Welch and guitarist Rob Ackroyd visited a 15-year-old in a Texas hospice center to play an intimate and emotional set for the young girl, Karinya, who is battling cancer. She was constantly asking the nurses if they thought she'd be well enough to go to the show, and they already knew the answer. So they contacted the band, who was happy to help.

Here's another version of the emotional video: