The 38-year-old fashion icon helped a guest restyle her hairdo after she suffered a malfunction with her 'magic sponge', a hair accessory used to create a bun.
"The girl's hair had started coming loose from her bun and one of her pals mentioned that you could see her 'magic sponge'. Kate took pity on the poor girl who was struggling to fix it and took matters into her own hands," a source said.
"She told her, 'I can't let another woman go out there with her sponge showing.' Then she grabbed hold of the girl, set her in front of the mirror and whipped out all the hairpins holding her hairdo in place and started again from scratch," the source added.
Later that evening, Moss, who attended the Marie Curie Cancer Care fundraising dinner with her husband Jamie Hince, helped English guitarist Ronnie Wood auction his self-portrait for the good cause, before taking to the dancefloor.