As a general rule, you should have a police report that you have been defrauded by fraud and your solicitor or tax adviser will gather all the necessary documents.
Your tax return will have to be amended as a result of this incident.
I am not sure that the loss from this fraud will be a tax loss for you.
Good point. What is necessary is to report it to the police, without that they will not believe him. And with this, although it is not 100% sure that they will believe him, I think it will be quite sure that he will avoid problems. In a normal country where the Treasury is not very inquisitorial, once the report is made and presented, if the Treasury does not want to believe him, and therefore not accept the deduction, you should present some kind of evidence against him, which in this case it seems that they will not get.
I don't know US law, but the general rules are that a taxpayer's money they lost to fraud will not go into a tax deduction. It is not a loss by trading or investing.
Therefore, the coins that this user lost as a result of fraud, reduces his assets on his tax return.
And if his tax office doesn't believe him, they will have to do their own investigation and request materials from the police. But any fraud has to be registered by the police, otherwise it's words without evidence.