Having crossed the US border many many times by land and air I would say that the woman did something to piss the inspector off. They have discretion and I have benefited from it more than once. Quick tip, the right answer always includes "Sir", admit a mistake and never argue.
If she did use Global Entry it may well be the fine is automatic. As others have pointed out it is an honor system, you tick the boxes and in this case she effectively lied and that will piss the inspector off big time. It would not surprise me if her Global Entry was cancelled. When you sign up there are plenty of warnings what will happen if you cheat. At $500 she was lucky. I'm surprised she was not strip searched.
EDIT - I see her Global Entry was cancelled. No doubt she is flagged in the system and I'd suggest clean underwear for the next international trip.