When real estate entrepreneur Aby Rosen wanted to restore his Gramercy Park Hotel to the forefront of New York's trend-setting scene, he invested in new paint and lighting, then turned to his wife, Samantha Boardman, for some fashion advice.

Which designer, he wondered, should create the wait staff's uniforms?

When Ms. Boardman's answer debuts on Monday at the hotel's Rose Bar and Gramercy Terrace, it will be one more detail the hotel can tick off in its drive to please an exacting, influential clientèle.

Instead of a design megastar, Ms. Boardman suggested Sophie Theallet, whose work is like a secret handshake in fashionable society—not widely known to the mass market but closely followed by a few. Her clothes are feminine, often sexy but always understated, with details like handwork and hidden hooks that are hard to appreciate from afar. "Sophie is a darling of a lot of people in the fashion industry," says Mr. Rosen. "More credit to my wife in choosing her."

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