The rock band fired up the crowd with the surprise rendition, after Gallagher stormed off complaining that his voice was "absolutely smashed".
In an interview with New Zealand TV Guide, Walker says he will now live his "wildest dreams".
Sheeran caught up with TVNZ1’s Seven Sharp to discuss his feelings on the mural.
It is understood Walker is battling a rare cancer-causing gene that runs in his family.
Young Nariah and mum Jess were devastated their tickets were fake, until Alison and Carmen came to the rescue.