All I can say are all the people in the thread that think he has no obligation to return the property are dead wrong. i've seen many people are the mistaken belief that unless the law is written down and specific you can operate outside of it. That is a mistake. They are confusing malem in se and malem prohibitum.
When you join the bitcoin system you agree that spends are permanent and non-refundable. You essentially agree that any spends made from your account in error are all on you. It's like if you go to work for a company that requires a confidentiality agreement and then you go to their competitor and tell them all you know about how the other company does business and they find out about it and sue you. You can't claim 1st Amendment, freedom of speech as a defense because you signed away that right when you took the job and put your name on the confidentiality agreement.