I believe knowing questioner will equip you better to answer this question. In Islam there is no concept of freewill, original sin or God’s son. All prophets/messengers/angels are sinless, every human being born is a sinless and God will and can never have a son. Saying Jesus is the Son of God is one of the greatest unforgivable sin.
Now about answer- Muslims firmly believe that the Quran is the last revelation from Allah and only good reliable word of God. They assume all the books that came before Quran are corrupted. To regard the Bible as true revelation from God, falsify Quran.
May be you can ask them, if they have ever read the Bible? Why do they think the Bible is corrupted? When do they think it was corrupted and what was the reason for corruption?
How do they know Quran is not corrupted?
You can mention following verses from Quran that talks about Torah, Psalms (Zaboor), & Injil (Bible)
In Quran 2:87 Allah declare this regarding Torah
“We gave Moses the Book and followed him up with a succession of messengers,”
In Quran 4:163 Allah says this about Psalms (Zaboor)
“We have sent thee inspiration, as We sent it to Noah and the Messengers after him: we sent inspiration to Abraham, Isma’il, Isaac, Jacob and the Tribes, to Jesus, Job, Jonah, Aaron, and Solomon, and to David We gave the Psalms,”
And Quran 3:3 Allah calls the books before the Quran a guide to Mohammed Gospel (Injil)
“It is He Who sent down to thee (step by step), in truth, the Book, confirming what went before it; and He sent down the Law (of Moses) and the Gospel (of Jesus)before this, as a guide to mankind, and He sent down the criterion (of judgment between right and wrong),”
Some other verses about the books before Quran are as follows:
“And in their footsteps We sent Jesus the son of Mary, confirming the Law that had come before him: We sent him the Gospel: therein was guidance and light, and confirmation of the Law that had come before him: a guidance and an admonition to those who fear Allah,” (5:46).
“Rejected were the messengers before thee: with patience and constancy they bore their rejection and their wrongs, until Our aid did reach them: there is none that can alter the words (and decrees) of Allah. Already hast thou received some account of those messengers,” (6:34).
“The word of thy Lord doth find its fulfillment in truth and in justice: None can change His words: for He is the one who heareth and knoweth all,” (6:115).
“For them are glad tidings, in the life of the present and in the Hereafter; no change can there be in the words of Allah. This is indeed the supreme felicity,” (10:64).
Answering Islam is a great website to know more about how to answer Muslims.
I hope this will help you in forming your answer. Above all, PRAY and seek Holy Spirit to lead you in your conversation. The target is not to win the argument but win your friend to Jesus Christ. I pray that God will reveal His truth to your friend. Amen