The third author of “Mastering the Art of French Cooking” has never been written about as much as Julia or Simcha, and no online source that I know of has either an obit or confirmation that she’s still alive–she’d likely be a centenarian. She might have fallen off the media radar if she later remarried and changed her name. Do any Dopers know more?
I know from Julia Child’s My Life in France (what a truly wonderful book, by the way) that Louisette’s second marriage was to Comte Henri de Nalèche, but I was no more successful than you at poking around the internet for more info.