Ms. Duwaji, who avoided the spotlight during her husband’s campaign and grants few interviews, had slipped into the party discreetly, opting not to be photographed on the red carpet, which saw appearances from celebrities like Martha Stewart, the Dare and Ego Nwodim. Wearing a leopard-print jacket and black boots, Ms. Duwaji nursed a beverage as she hung out with a small band of friends. She turned away from photographers but warmly greeted young New Yorkers who approached her, as a security detail kept watch.