Soldato
An adult is responsible for their own seatbelt not the driver, however letting the thick numpty hang out the window should come under dangerous driving.
What can the driver do? Tell her sternly to sit down and face the front. If she refuses, it's not like he/she can pull over on the M25. If he/she could, then what? Ask her to leave the car. If she refuses, he/she can't put a hand or gently pull her out.