And, what is to keep the friend from simply keeping the car? If his name is on the registration and the title, there's no way that your dad can FORCE him to give up the car absent a court order. Sure, dad can find out where it is, hire a tow truck (or use a key if he has one) and snag it, but the friend can do the same thing. That sort of tit for tat can get tedious.
My recommendation would be for dad to try and convince the friend to transfer the registration and title to dad, and when the friend has fulfilled the contract and paid the agreed upon price for the vehicle, then he will sign the title over to the friend.
So long as the friend's name is on the registration and title, he doesn't have to do anything and he can still retain control of the car.