While I have not been asked directly, I know that many devout Muslims wonder about the passage in the Qur'an that explains that Jesus was not dead, but that he was taken up to heaven. That section of the Qur'an is very strong in stating this, and I know that it makes many Muslims wonder how Christians could be so in error on this point.
Here is the answer. God, in his wisdom, is always able to anticipate how humans will behave and the depth or weakness of both their current and future faith. God has spoken in three waves, you could almost think of it. He spoke to the ancient Israelites and gave them the Law. He sent Jesus Christ to be the Messiah, but remember, much of what Jesus preached (and later the Apostles) are quotes from the Old Testament. Jesus is not the second wave that replaces the first wave of direction from God, but the fulfillment of the Messiah and Savior that was prophesied. Why was Jesus sent at all? Because God knew even as he instructed humans during the ancient Biblical times that they would stray and not understand their covenant with God unless the Messiah were sent. Likewise, even as Jesus walked the earth and preached, of course God knows that some will believe and some will not. But even more importantly, God knows how future generations will both stick to the truth, but also many will spread lie and heresy. God knew, of course, being all wise and seeing human actions far into the future, that someday certain "Christians" will claim that Jesus was only a man who died. And that is exactly what we have seen, where people who are claimed or nominal "Christians" diminish Jesus as being a wise and gentle philosopher, but who died and then his "cult" made up fairy stories about him. This is an amazing libel against Jesus and God himself, but God knows humans much better than they do or ever will themselves.
So when God decided to instruct the Prophet (PBUH) in addition to explaining the faith and the roots of Islam back to the Abraham fount, and even to Adam, and the share in salvation that those who embrace Islam have, he also graced Islam with additional information about Christianity in order that Muslims can be part of safeguarding the truth about Jesus Christ. (An example of the additional information graced to Islam is the speech of the baby Jesus as an infant declaring himself. It is not a coincidence that Gabriel, who announced the conception of Jesus is also the angel chosen by God to instruct the future people of Islam.) This is why God advised the Prophet (PBUH) that Jesus did not die, but resurrected, as direct refuting of the libel that Jesus was just a man who died. Look how even recently Hollywood fakes try to exploit this disbelief in order to claim finding "the tomb and bones of Mary and Jesus and the family." So God advised the Prophet (PBUH) that Jesus did not die, meaning he was not a dead body and buried until his body turned to dust and only bones remained, and there was not resurrection, as heretics like to claim. Jesus did die for three days. During this time he went into the shadowy "hell" where forefathers in their error had to remain until his coming, and in those three days he led them to heaven. Catholics recite this knowledge in their prayer The Creed. Then he returned to his body (bearing the same marks that it had, although now he was glorified in his body) to teach another forty days before ascending. So when Muslims are exhorted to stand firm on this point, the point that they are being graced to share in the defense of the faith is that Jesus did not perish in human body form.
Bless all who hear and believe.