To do so, it's helpful to keep a couple things in mind.
First, remember that the typical Muslim is what we might call a "folk Muslim" or "cultural Muslim." That is, she or he is not likely to be a scholar of either the Qu'ran or the Bible. Their views are shaped greatly by the teachings of an Imam or cleric and tradition. So, don't be afraid of hearing this charge.
Second, a few key passages from the Qu'ran can be used to suspend belief in Muslim claims about a corrupted Bible. The Qu'ran everywhere affirms that the Law of Moses (Torah), the Psalms of David, and the Gospels were "revelations" from God. For example:
"No just estimate of Allah do they make when they say: "Nothing doth Allah send down to man (by way of revelation)" Say: "Who then sent down the Book which Moses brought?- a light and guidance to man..." (Surah 6:91).
"And it is your Lord that knoweth best all beings that are in the heavens and on earth: We did bestow on some prophets more (and other) gifts than on others: and We gave to David (the gift of) the Psalms" (Surah 17:55).
"The Great Terror will bring them no grief: but the angels will meet them (with mutual greetings): 'This is your Day,- (the Day) that ye were promised.' The Day that We roll up the heavens like a scroll rolled up for books (completed),- even as We produced the first creation, so shall We produce a new one: a promise We have undertaken: truly shall We fulfill it. Before this We wrote in the Psalms, after the Message (given to Moses): My servants the righteous, shall inherit the earth" (Surah 21:103-105).
"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. Let the people of the Gospel judge by what Allah hath revealed therein..." (Surah 5:46-47).
Notice in the last quote (Surah 5:47) that "the people of the Gospel" are told to judge all things by what is put in the Gospels. As Christians, we should seize that as an open opportunity to move directly to Jesus. Moreover, we should our Muslim friends to this "revelation" in the Qu'ran, where even in the time of Muhammad the Gospels were still seen as reliable enough to arrive at the truth. Surah 10:94 read: "If thou wert in doubt as to what We have revealed unto thee, then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt (see also 16:43; 21:7). Even the writers of the Qu'ran admit in places that the Bible is sufficient for matters of faith and conduct.
Further, this key principle runs throughout the Qu'ran: "There is nothing that can altar the words of God" (6:34; 10:64; 18:27). According to the Qu'ran, Allah promises to watch over the revelation and guard it from corruption (15:9). These sentiments find their expression in the Bible first. "Your word, O Lord, is eternal; it stands frim in the heavens" (Ps. 119:89; 1 Pet. 1:24-25). And Jesus taught "It is easier for heaven and earth to disappear than for the least stroke of a pen to drop our of the Law" (Luke 16:17) and "Heaven and earth will pass away, but my words will never pass away (Matt. 24:35).
Let us be quick to ask our Muslim friends:
- If the Bible had been corrupted would the "perfect" Qu'ran have confirmed it?
- If even the Qu'ran teaches that the Torah, Psalms of David, and Gospels are words from God, and both the Bible and the Qu'ran teach that god protects His word and that His word will not be corrupted, then shouldn't we assume that the Bible is reliable and true?
What does it say about your view of Allah/God, then, if his word is corrupted by men? Either you must conclude that Allah is not omnipotent being unable to guard his word from mere men, or that he is not a god of truth because he willingly allows men to corrupt his revelation.
At this point, you're praying that the Holy Spirit of God makes your friend honest and willing to suspend his disbelief in the Bible in order to hear a clear presentation of who Jesus is from the Law, the writings, and the Gospel. May the Lord gives us much fruit from our labors!