Mary was doing a little food shopping after work at Westside Market and saw Hayley Mills. Mary gave her a knowing nod and smile, and Ms. Mills smiled back. She is still quite pretty (and doesn't appear to have had any cosmetic surgery). IMDB says she was born in 1946.
Jeff wondered how Mary recognized her. Well, she played a character named Miss Bliss in a few episodes of a show Rachel used to like to watch when she was little, called "Saved by the Bell." By the time Rachel saw those episodes, they were in reruns--but nevertheless, that's how Mary recognized her.
IMDB also says she is in a current show called "Wild at Heart" on BBC, which is fitting since Ms. Mills is British.
But for most of us, Hayley Mills will always be Sharon and Susan in "The Parent Trap," singing "Let's Get Together," and Pollyanna from the movie of the same name. Mary also remembers her from "That Darn Cat!" with Dean Jones.