Is a person sinful due to the fear of dying as a non-Muslim, and doubts about the validity of the religion, and the questions that Satan casts into the soul? Similarly, is his friend sinful due to Satan's whispers about the essence of God, despite their efforts to overcome these doubts?

Satanic whisperings regarding God's creation do no harm as long as the heart is tranquil in faith. One must seek refuge from Satan and say "I believe in Allah" when such whisperings occur. The fear of Allah and His punishment is good in its origin, but it should not exceed its limit to become an illness. Rather, it should be accompanied by hope and good presumption of Allah. "Certainty" (Al-Yaqeen) in the verse (until there comes to you the certainty) means death.