Why does Madame Forestier fail to recognize Mathilde?

After losing Madame Forestier's necklace, the Loisels had to take out tremendous loans in order to purchase another such necklace—as they believe her necklace to have contained real diamonds (when it did not). They spent the next ten years working themselves to the bone in order to pay back all the money they borrowed. The narrator says,

Mme. Loisel looked old now. She had become the woman of impoverished households—strong and hard and rough. With frowsy hair, skirts askew, and red hands, she talked loud while washing the floor with great swishes of water.

Mathilde Loisel has had to do a great deal of manual labor, has had to give up her dreams of mixing with the upper crust, has had to give up the servant she once had, and has essentially had to become like a servant herself.

She came to know what heavy housework meant and the odious cares of the kitchen.

Mathilde has washed dishes, done laundry, taken out the garbage, fetched water, done her shopping, haggled with merchants, and in general tried to save every penny she could. Such a life, in addition to being hard on the body, would be quite hard on the mind; worrying about money all the time is incredibly stressful, and stress takes a major toll on a person, both mentally and physically. Therefore, her old friend does not recognize Mathilde because those ten years of hard labor have aged her a great deal more than those same years have affected her friend, who still appears young and beautiful.

When Madame Forestier bumps into Mathilde on the ‎Champs-Élysées, she initially doesn't recognize her. Mathilde appears to have aged prematurely, and her clothes are all haggard and worn. After the unfortunate incident with the lost necklace, Mathilde and her husband had to take out a number of loans to pay for a replacement. This crippled them financially, plunging them into dire poverty. Mathilde, who once had such wonderful dreams about mixing in high society with the social elite, has been forced to work for a living, toiling away each and every day just to make ends meet. This has taken a serious toll on Mathilde's physical health and appearance. She's not the same woman she once was, and so it's not surprising that Madame Forestier doesn't recognize her.