If you believe that the nephew defrauded you bank and that he may deserve criminal punishment, you should contact the police and describe what happened, providing any and all supporting evidence. They may not agree that it was a crime--they may, for example, think that it was a mistake or misunderstanding, which can lead to civil liability, not criminal (see below)--but when you want to press charges against someone, contacting the police is always the first step.If your father feels defrauded by whatever amount he did not receive and the nephew will not make good that amount, your father could sue your nephew in small claims court (there is a  small fee, so your father would need to make sure its worth it) for the amount  owed.