No, it is gravity. In the scenario above, she would have had less drag than him. The benefit he got from being behind is a big factor of course (especially at only 1-2%) but heavy things definitely do not fall at the same rate as light things.

Not until you reach terminal velocity and that would be higher for her.

edit: I am not posting before 5AM again. Of course mass isn’t the cause of a heavier rider rolling downhill faster (doh!), but the lower drag force relative to the graviational force as per the comment above about not being twice the frontal area.