Just a week after revealing that she was back in the studio, Lily joined her pal midway through his set to perform Just Be Good To Green.
According to the Daily Mail, the rapper had given his Twitter followers a hint of things to come earlier in the day, writing: "If @lilyrosecooper arrives here before I'm due on stage should we try to persuade her to join me?"
And nearly two years after her last public performance, Lily was back on stage wearing a Stone Roses T-shirt in tribute to the headliners.
The Mail reports that she later joked on Twitter: "No @professorgreen wasn't swinging a cat around on stage @heatonpark that was me trying to sing. loved it, thanks manchester x x x."
Until recently, Lily had been taking a break from the music business to concentrate on being a mum to daughter Ethel.