Reference: http://www.alfawzan.af.org.sa/node/16888
Question: Is there any expiation for the one who has died whilst having prayers and fasts which he had not made up for? And what [form does this] expiation [take]?
Response: The prayer is not to be made up; it is not [legislated] for anyone to pray on behalf of another. As for the fast, then his representative, or next-of-kin should fast on his behalf, as he ﷺ said:
«Whoever dies and had some [obligatory] fasts [he had not made up], [then] his representative should fast on his behalf.»
…and [it has been reported] in another narration:
«Whoever dies and had some fast [he had not done] for an oath, [then] his representative should fast on his behalf.»
[So to conclude], it is the fast for which someone can do on behalf of another. As for the prayer, then no-one can do so on behalf of another.
