Is it permissible for a Muslim to say, concerning the death of a Christian or a disbeliever: "May Allah lighten his burden"?

There is no harm in supplicating for a sick Christian for healing or for worldly benefits, because it is permissible to supplicate for a non-belligerent disbeliever for worldly advantages.

However, it is not permissible to supplicate for him at the time of death to lighten his suffering, because the soul of a disbeliever is seized with severity, without gentleness or leniency. This is indicated by the Prophet's (peace be upon him) narration about the departure of the souls of the believer and the disbeliever, and by Allah's Almighty saying: "And if you could see when the angels take the souls of those who disbelieved, striking their faces and their backs and [saying], 'Taste the punishment of the Burning Fire.'" (Al-Anfal: 50), and His Almighty saying: "If you could but see when the wrongdoers are in the agonies of death while the angels extend their hands, [saying], 'Discharge your souls! This Day you will be awarded the punishment of [extreme] humiliation for what you used to say against Allah other than the truth and [that] you were arrogant toward His verses.'" (Al-An'am: 93).