Is talking about immodest women after they have left the place where they were present considered backbiting, and does this fall under the saying: "Whoever slanders will be afflicted"?

Speaking about people's faults and shortcomings is backbiting (gheeba) if it is merely for the sake of relaying information or gossip. However, it is not backbiting if it serves a legitimate Shariah interest.

There is no harm for a woman to wear in front of other women what does not reveal her unlawful 'awrah.