According to me giving keys would be a sort of confusing thing and a lot of problems would occur if u just gave the car keys and then that person died or crashed. I think just using the /e (toggle engine) command like the /lock command would solve the problem. Just type /e from the same distance as of locking the cars and the engine will start. So in a completely locked and switched off car, use /lock or MMB to unlock it and then /e to start engine of the car so that other person can just get in and start driving it.