Why help the petroleum companies make profit and build up extra km's on your engine for nothing.
If he has insurance and the car has been reported stolen let the cops and the insurance take care of it. If you spot it report where you spotted it and I can guarantee the cops won't be onto it and it will be 10 suburbs away since you reported it.

The car was stolen and the owner was fooled by the buyer to have his car taken away that's a big fail on his behalf, obviously he won't be letting anyone test drive a car again without photographic ID or a snap shot behind the steering wheel and the keys out the ignition barrel when you're out.
I don't care who it is when they want to purchase a car off me I ask them for some valid ID before they are behind the wheel of my car, if they refuse to they can go and screwed.
Your car, your duty of care to keep it in safe hands when you have someone looking at buying it. It's not hard unless they pulled a knife on you than it's a different story.

Hoping it is found in one piece for old mate and that he will know what to do if he ever deals with buyers again.