The deceased had missed numerous Salaat.

Answer

If the deceased had made wasiyyat (bequest) to pay the Fidyah for his missed Salaat, then it will be Waajib to pay it from one third the value of his assets. If the Fidyah amount exceeds one third the value of the estate, then paying the excess is not Waajib. Nevertheless, the heirs may pay the excess from their own wealth.

If he had not made wasiyyat, then it will not be Waajib on the heirs to pay the Fidyah. Nevertheless, it will be best to pay it and to make dua for the maghfirat of the deceased. In this case, that is, when a wasiyyat was not made, the heirs may contribute whatever they can afford from their own wealth.