what does Madame loisel learn from Mme. Forestier when she runs into her on the street a decade later?

Mathilde Loisel is the protagonist in Guy de Maupassant's "The Necklace." Though she is both "pretty and charming," she does not have the wealth nor the lifestyle that she desires. Upon receiving an invitation to an important social affair, Mathilde spends money on a dress because she feels she has nothing worthy. Since she cannot afford to purchase a necklace, she borrows one from her friend, Mme. Forestier. Mathilde finds, "in a black satin case, a superb diamond necklace," which her friend easily agrees to loan her.
When Mathilde and her husband return home from the social affair, she realizes she has lost the necklace. She decides not to tell her friend but instead decides to purchase a similar necklace to give to Mme. Forestier as the original. Mathilde and her husband borrow a large sum of money, thirty-six thousand francs, to purchase a new necklace. It takes ten years to repay the debt.
After repaying the debt, Mathilde sees her friend, Mme. Forestier, and tells her the truth about the necklace. She informs Mme. Forestier that she and her husband spent ten years paying for the diamond necklace. Mme. Forestier is surprised, telling her, "But mine was imitation. It was worth at the very most five hundred francs!" Mathilde learns that she and her husband spent thirty-six thousand francs when the necklace was only worth a fraction of that amount.