If you give in she wins and holds you by the nads every time you want to implement a policy.
Also no more lettering on cars for your sales force of course.
I'd have her rent another car but designate a fixed amount for monthly payments, maintenance, car wash and insurance, she can rent a Ferrari if she wants for her country club rides. Keep paying the gas but the amount for peripherals would be fixed. Make this available to all your sales team. I doubt they will take it.
That's what I'd do. My humble opinion.