1. Ask the person who fraudulently uses the signature to immediately correct, restore and destroy the file whose signature is fraudulently used.
2. If the reputation is affected, you can ask the other party to restore the reputation, eliminate the influence, apologize and compensate for the losses.
3. If the other party's fraudulent signature act is suspected of fraud, it can report the case to the public security organ, which will handle it according to law after investigation.
It is difficult to define whether the fraudulent use of signatures is illegal, because the circumstances are too minor, there is basically no provision in criminal law, and there is no operational punishment in civil law. If a lawsuit is filed, the object of the lawsuit should be compensation rather than infringement. This receipt belongs to the category of evidence. You need to identify it and verify the authenticity of the receipt and its contents by seeing if it is your father's handwriting. This kind of appraisal is usually done through the court.