Dance group MerseyGirls entered the ITV talent contest in 2017 - but it was due to be their last project together.
Member Julia explained that she would subsequently undergo surgery to correct her scoliosis, leaving her unable to dance.
Moved by their story, Simon personally paid for Julia to have a more advanced operation that would not rob her of her talent.
She flew to America for the £175,000 procedure shortly after the series final, and was back dancing within months.
The media mogul's life-changing gesture came full circle tonight when MerseyGirls returned to the BGT stage.
The Wirral five-piece are one of the acts taking part in The Champions, a spin-off series featuring past contestants competing against international acts for ultimate glory.
After they performed in Saturday's second heat, Simon said: "The fact that you've stayed together and supported one another, and were able to come back, It's everything."
Julia replied: "I thought that now I would be at home not dancing for the rest of my life. Here I am backflipping with my best friends. Simon, thank you so much. I don’t think I have ever been so happy as I am right now."
Speaking about the reunion during filming, Amanda Holden revealed: "I felt enormously proud of Simon as he is extremely modest and when I brought it up he did not want to discuss it.
"This is one of the things I admire most about him, he never does any singing and dancing about the amount of good work he does with charities behind-the scenes.
"The girls were absolutely delightful and the minute she did a backflip we basically all burst into tears, it was like a miracle."
Alesha, who had pressed her golden buzzer for the group in 2017, added: "I’ve actually been following Julia’s progress since she first auditioned on the show. She always sends me updates and things she’s been up to. I’ve been quietly keeping an eye on her and actually saw a video of her a couple of weeks ago doing a flip that was the first time she was able to do it since having the operation.
"I felt quite emotional seeing her perform. It was something quite special that she came on the show needing that operation, Simon paid for her to have it done and now she’s back on Champions performing, something she feared she would never be able to do. They are such a special group of girls and the friendship they have is so gorgeous.
"It feels like a full circle moment, you meet these people, hear their stories first time around but then life and the show moves on. So for the audience to be reminded of what they have been through and see how far they’ve come since the show, it’s a special thing. It’s the heart of the show."
Britain's Got Talent: The Champions airs Saturday nights on ITV.