Not that I want to get dragged into this and arguing with MD seems to be a bad move, but having said that: Boys gets fired all the time without HR cringing, girls on the other hand is often a recipe for a lawsuit.
The question isn't whether bad pilots should be fired, they should, but since it is a male dominated bussiness, it is easy to think gender discrimination had something to do when a female pilot is let go, even if it was for subpar piloting skills.
Sort of akin to divorces, the mother could be smoking crack to high heavens etc., yet often the courts give her custody.
Of course, if the story of this female F/O really is true, then it would seem a nice little visit with the chiefpilot would be in order.