“Do I have a right to know where you are going in your private car?”
The FAA strongly encourages flight plans although you’re free to fly without one. If she flies without one then she can have total privacy.
It depends on the flight: If the flight is over 18,000 feet altitude a flight plan is required.
“If she flies without one (a flight plan) then she can have total privacy.”
As with a number of other things, you know nothing about aviation, i.e. don’t know what you’re talking about.