Can someone pay Fidya for Salat of a dead person who missed some Salat in his life?
Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no Fidya (ransom) to be paid on behalf of a dead or alive person who missed some prayers.
But, if the dead person did not give up performing prayer during his life, then it would be beneficial - Insha Allah - to do some good deeds and to ask Allah to grant him the reward of those good deeds.
For example, alive person can give money as a charity, and then, pray to Allah to grant him (the dead one) the reward of this charity. According to some scholars, it will be beneficial for the dead, if alive person presents him the reward of any good deed, even prayer.
Thus, depending on the above opinion, alive people can pray with the intention of granting the reward to a dead person.
Allah knows best.
You can search for fatwa through many choices