Is it permissible to assist the mother of the slain person financially to find out who killed her son, given that her son had previously killed his wife and was imprisoned, then he was killed by the brother of his slain wife, and the mother of the slain person is seeking Allah's law?

There is no retribution (qisas) upon the father if he kills the mother of his children; rather, it is waived for him. This is because if it were to be due for his child, and a child is not subject to qisas against his father, then how could it be due for him when he commits a crime against someone else? For this reason, the wife's brother is not permitted to kill the husband; his case should be referred to the Sharia court.