What must the witness do after having falsely testified that the deceased has no heirs other than his father, thereby depriving his two grandmothers of inheritance, knowing that the deceased's father is poor and destitute?

False testimony is a major sin. If the father has already given the grandmothers their share, then you must repent and seek forgiveness, and you are not required to go to court. However, if the father took the money for himself, then you must try to convince him to give back the rights. Otherwise, you must go to court and recant your testimony so that the rights are returned to their rightful owners. There is no expiation for false testimony except repentance and the return of rights.