There can be no doubt that Qisas is a common right between the Lord (Glorified and Exalted be He) and the heirs of the victim. As a result, the heirs of the victim or any of them can pardon the killer and demand no Qisas. In case one of them pardons the killer, the claim to Qisas will be relinquished. In that case, the heirs have the right to demand nothing but the Diyah. Allah (Exalted be He) says:
Al-Qisâs (the Law of Equality in punishment) is prescribed for you in case of murder
until His Saying:
But if the killer is forgiven by the brother (or the relatives, etc.) of the killed against blood-money, then adhering to it with fairness and payment of the blood-money to the heir should be made in fairness. This is an alleviation and a mercy from your Lord.
It is permissible for you
to intercede and ask for pardoning the killer, since the right to demand Qisas vests in the heirs of the victim.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.