How did Mme Forestier recognize Mme Loisel when they met in the Champs-Elysees?

Madame Forestier did not recognize Mathilde.... she had no idea who Mathilde was.

She went up to her. "Good morning, Jeanne."
The other did not recognise her, and was surprised at being thus familiarly addressed by a poor woman. "But . . . Madame . . ." she stammered. "I don't know . . . you must be making a mistake."
"No . . . I am Mathilde Loisel."
Her friend uttered a cry. "Oh! . . . my poor Mathilde, how you have changed! . . ."