Isn’t she also Her Royal Highness?
She’s been around forever. She was still around when The Beatles were singing about Her Majesty.
I just think hearing some of her stories about her father, WW2, Churchill, Eisenhower, De Gaulle, Kennedy, etc... would be fascinating beyond belief.
HRH is used for members of the royal family except for the monarch. She was HRH Princess Elizabeth before she became Queen.