Five Day Lover (1961)

During Madeleine's fashion show, Claire meets Antoine and becomes his mistress. Since she's married to George, a wealthy man with a modest job, she only spends a few days a week with Antoine, who is also rich, often taken to meeting by his chauffeur Marius--and also the lover of Madeleine, Claire's best friend. Everything works perfectly until Madeleine discovers the truth. She prepares her vengeance and organizes a celebration which she invites Antoine, Claire, and George.