Hi --- The £1500 cash has been recorded as changing hands to which your friend has a receipt for, which means the sum of money was exchanged voluntarily and is a civil matter to which private legal action would be needed regarding the final outcome, should things go wrong.
The old car however is still legally registered in your friends name, that could become a criminal matter in which the Police would become involved with, if your friend does not either sign over the ownership, or the car is not returned as he requests.