One of the simplest ways of defending against the idea that the Bible has been changed is looking at the character and power of God. Usually these are two areas in which our Muslim friends would agree with us. Firstly, the fact that there is no one more powerful than God is something that all Muslims would claim, and something the Quran is explicit on. Quran 22.74 states clearly that "Allah is all powerful and exalted in might". As Christians, we believe too that there is noone more powerful that he is. This is stated clearly in verses like Isaiah 46:9-10 and Jeremiah 32:17.
This means that if anyone wanted to change or destroy his holy books, they would have to be more powerful than him. We all agree that there can be none more powerful than him, whether human beings, or angels and demons.
22:74They measure not Allah His rightful measure. Lo! Allah is Strong, Almighty. Al-Hajj 74
Secondly, God cares about his words. In many places the Bible affirms the fact that he cares about us and wants us to know him. See for example Psalms 55:22, Acts 17:26-27 and John 3:16.
The Quran too speaks of itself in terms of being protected and guarded by Allah (e.g. Quran 15.9 and Quran 85.20-22). If his words that were revealed in the past have been changed, it must logically infer that one of these facts are not true all the time. Either he is not powerful enough to protect his word or he does not care. The claim that Scriptures have been corrupted, ultimately raises serious questions about the character of Allah.
A common Muslim response is that the purpose of life is a test, and it is the duty of mankind to protect the word entrusted to them. With mankind having failed that test, God has now stepped in to protect his final revelation. As well as completely misunderstanding the purpose of scripture with its unfolding story of God's plan of salvation, this argument fails to recognise that the Quran categorically states that no one can change his words. (See Quran 6.34, Quran 6.115 and Quran 18.27).
The issue the Muslim has is, if Allah has allowed his words to change in the past, how does the Muslim really know that he has not allowed the Quran to be changed. How can any of his words be trusted? After all the Quran states that Allah is 'the best of schemers'1 (see Quran 3.54)2. This is an issue worth raising, especially as there are big questions over the reliability of the Quran. (see Is the Quran Reliable)
It is good to ask your Muslim friend whether they have read what Jesus says about the words of God. Good places to take them are John 10:35 where Jesus affirms the Old Testament and Matthew 24:35 where he states that his words cannot be changed.
Other useful verses are Isaiah 40:8 and 1 Peter 1:25, both of which affirm the unchanging nature of God's words. These verses show us that God's words and therefore God himself are completely dependable, as the one who is both supremely powerful and also loving towards us.
Why not ask your Muslim friend if they know that Allah loves them, and if so, on what basis do they think this? This could be an opportunity to explain how God demonstrated his love for us when Jesus died. (See: Romans 5:8)