non-heir

Non-heir discharging the fidya for the missed Salaahs of the deceased

Q: Previously I asked about a man who passed away but for the majority of his life did not pray Salaah. As per your previous reply, fidya can be given on behalf of the deceased: What do the Ulama state about a person who is not an heir but is related to the deceased, can they give the fidya on his behalf? Furthermore, how does one calculate the fidya per Salaah?