[As an adult, she learned that she was actually genetically female and began transitioning]
So, he learned that he was actually genetically XX when he had gone his whole life thinking he was XY? That’s a major screw-up by his doctors. And, why, if he was actually “genetically” a female, does he need to transition?
Nature can play cruel tricks sometimes. If she has XX and no Y chromosomes, she is genetically female no matter what she has between her legs.