I have heard that bounce dryer sheets will keep away mice and other rodents.
Driving the car should definitely scare them off and the dryer sheets should prevent them from coming back and also make the car smell nice.
One thing to note however the insurance companies where I live consider mice/rodent infestations of vehicles to be a biohazard. If you report this to the DMV they will write off your vehicle and you can probably get it's entire value back or maybe a bit more.
It may be worth it if your friend wants a new vehicle. It is definitely not a good idea to breath squirrel/chipmunk poop for too long.
+1 for the snake idea aswell.