Finding money and not giving it back to the owner or to the police is considered as theft in various countries. Today there was a report about a Romanian man who bought a cabinet and inside of it he found a hidden box with approximately 59.000 if I remember correctly.
The report was on a German channel and a lawyer explained that in Germany, if you buy an item from someone and find money inside, you have to give it back. Otherwise you could get a hefty fine and even go to prison. You are also entitled to a 5-10% reward depending of the amount found.
If you give the money to the authorities and nobody comes forward to claim the money after X amount of days - the money will be yours.
Sounds like he should have kept his mouth shut.
There would be no way to prove he found the money if he didn't admit it.
Same with bitcoins, really hard to prove he was the one that found them.
He could always send them to his new address before spending them and just claim he bought them with cash, denying that this address belonged to him.