A lawsuit over L'Oreal heiress Liliane Bettencourt's gifts of 1 billion euros ($1.5 billion) to a photographer friend will go to trial mid-April, a French judge decided, a move that promises to throw personal details of one of France's most private families into the open.

The lawsuit was filed two years ago by Ms. Bettencourt's only daughter, Francoise Bettencourt-Meyers, who accuses the photographer, Francois-Marie Banier, 62 years old, of exploiting Ms. Bettencourt's mental weakness.