The police???
No, this is a civil debt.
The only way the police would be involved is if he can prove a deception. That means the "mens rea", i.e the guilty knowledge at the time of the offence would be sufficient to prove that the purchaser had no intention whatsoever to pay for the car. In otherwords, impossible to prove!!!
A solicitor or the small claims court I am afraid is the only way forward.
However, civil redress could be awarded if he took the car back.
Yes, he does not know own the car as such, but a stone bonker defence to ALL theft crimes is "I thought I had a legal right to"
Dodgy grounds, but a possibility if he can handle himself.