What is the ruling on seeking Allah's mercy for and giving charity on behalf of a friend who, before his death, asked to be buried in the Muslim way, and was indeed buried in that manner?

Islam should have been presented to the Christian. If this was not done, there is no sin on the one who did not do it, because there is no doubt that he knew about Islam and heard of it. The default is that he died upon disbelief unless the contrary is known. Therefore, he is treated like disbelievers: no prayer is offered over him, he is not buried in Muslim cemeteries, and no supplication is made for him; due to the Almighty's saying: "It is not for the Prophet and those who have believed to ask forgiveness for polytheists, even if they were relatives, after it has become clear to them that they are companions of Hellfire." Visiting his grave is permissible for admonition and reflection. A Muslim burying a disbeliever is not permissible, as stipulated by the Hanbali and Maliki jurists, except in cases of extreme necessity if there is no one else to bury him other than a Muslim. As for the Shafi'is, they hold that it is permissible for a Muslim to bury a disbeliever, but not in Muslim cemeteries.