No, she can't: when the law is violated, the victim of the crime is only a witness (often called the "complaining witness") to the crime--she is not in charge of the criminal case, the way someone filing a lawsuit (the "plaintiff") is in charge of the civil case. Rather, the criminal case is brought by the state (the authorities) to enforce the law, and can be brought based on the complaint of anyone who witnessed or otherwise has evidence of a crime--in this case, the bank. Anyone who is aware of a crime may report it, and no one has the right to stop them from doing so. The bank may choose to not bring this to the police, if they haven't already, if your grandmother asks them--but they don't have to listen to her, and if they have already brough it to the police, it is effectively out of their hands.