In that version of the story, Martha Wayne survives her encounter with Joe Chill in Crime Alley but goes crazy when she sees Bruce Wayne’s dead body lying on the ground — turning her into Joker, and turning Thomas Wayne into Batman.
Female Jokers have popped up over the years (albeit not as frequently as male Jokers), with Martha Wayne’s Flashpoint Joker arguably becoming the most iconic female incarnation of the character.
Before the purists out there get all worked up over the prospect of changing Joker’s gender, it’s worth mentioning that there’s precedent for this possibility in DC lore.