Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother in Islam, thanks for this interesting question, which reflects part of the misconceptions that some people have about Islam. We really commend your efforts in pursuit of truth.
This is what is required of all people, to seek truth and not to give in to anything that may cloud their minds with false ideas about Islam.
The Qur’an was written down in the lifetime of the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) used to ask some of his Companions to record the verses as they were revealed.
In addition to this, many of the Companions used to learn the Qur’an by heart. Thus, although the Qur’an was compiled in one volume after the Prophet’s death, it was written down during his life.
Responding to your question, Dr. Sano Koutoub Moustapha, professor of fiqh and its principles, International Islamic University, Malaysia, states,
Thank you and may Allah grant you more knowledge. You were very right in what you told your classmate. However, you should differentiate between writing and compiling. The Glorious Qur’an was compiled after the death of the Prophet (peace and blessings be upon him); that is, it was fully written and protected at the time of the Prophet and it was also fully memorized by many people at the time.
In addition to that, Allah promises that He will protect this book against all forms and types of destruction and cheating. This divine promise has always been there.
Thus the Glorious Qur’an is the only book which has not been subject to falsification or amendments. Furthermore, the first compilation of the Qur’an occurred only less than three years after the death of the Prophet, not after ten years as your friend mentioned.
All in all, the Glorious Qur’an has been proven throughout history to be an accurate and consistent book of God. All attempts to amend it or falsify its contents have failed. Yet, the Qur’an does not contain any teachings and commands that go against the nature or the interest of mankind.
Finally, do request your friend to go through the Glorious Qur’an. Surely, he will find it a wonderful and interesting book.
Allah Almighty knows best.