Is it permissible to supplicate for mercy and forgiveness for a Christian person who died, after they requested that?

It is not permissible to seek forgiveness for a Christian friend who died in shirk (polytheism), for the Shariah has indicated the prohibition of supplicating for forgiveness and mercy for one who died as a disbeliever. The Prophet, peace and blessings be upon him, was forbidden from seeking forgiveness for his uncle Abu Talib. The scholars have unanimously agreed on the prohibition of seeking forgiveness for one who died a polytheist, and Christians are polytheists.