Mme. Forestier is mentioned anonymously as the "rich friend" whom Mathilde knew back in the convent days. Apparently Mathilde feels too jealous to see her much.
Following her husband's idea, Mathilde visits Mme. Forestier to borrow jewelry to go to the ball.
Mme. Forestier shows Mathilde various jewels, which leave her unimpressed until she finds the diamond necklace. Mme. Forestier lets her borrow it.
Mme. Forestier gets the necklace back from Mathilde more than a week after the ball. She's miffed because it has taken so long to get it back. She also doesn't seem to notice that her necklace has been replaced by another one.
Mathilde meets Mme. Forestier strolling down the Champs Elysées with her daughter one fine Sunday afternoon, a little more than ten years after the fateful party.
Mathilde comes clean spills her beans about the necklace, that it was replaced.
Mme. Forestier spills her beans about the necklace, that it was a fake in the first place.