Is it permissible to say that Allah may have mercy on some disbelievers and admit them into Paradise because of their good morals and righteous deeds, and is it valid to use the story of the prostitute who gave water to a dog as evidence for this, and which is more severe: polytheism (shirk) or presuming to speak for Allah (al-ta'alli 'ala Allah), even though both invalidate deeds?

What the questioner told his friend is true and consistent with the Qur’an and Sunnah. Whoever dies as a disbeliever will not enter Paradise, nor will Allah’s mercy encompass them in the Hereafter, nor will Allah forgive them. This is based on His (the Almighty’s) saying: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills," and His (the Almighty’s) saying: "Indeed, those who disbelieve and die while they are disbelievers – never will there be accepted from one of them an amount of gold filling the earth, if he would ransom himself thereby."

As for the story of the prostitute who watered a dog and Allah forgave her, it does not imply that she was a disbeliever and Allah forgave her disbelief. Rather, she was a prostitute, and Allah forgave her fornication because she watered the dog. Some scholars have clarified that her action was an indication of the strength of faith and the firmness of monotheism in her heart, and that was a reason for forgiveness.