First you will need to prove the existence of God and then work your way to explaining the reliability of Christianity. But to say the very least you can not give 2 dollar answers for a 2 million dollar question. So please feel free to continue to ask questions in this thread as your friend would in a discussion.
First of all, I would explain the fact that we as humans communicate by talking, writing, and spending time with one another. That is how we are created. So, if we are created in that way, it only seems reasonable that our Creator would communicate with use in that manner, and He has. First of all God has communicated with us verbally, and sometimes He still does, but He certainly did with Moses, and the prophets. God has communicated with us through writing, we have the Scritpures. And God has also come down on earth and spent time with us, Jesus Christ who is God in the flesh.
But as for the Bible, my 1st approach would be the vast amount of ancient Bible manuscripts. We have over 5000 Greek manuscripts of the New Testament alone. Over 10,000 ancient Latin manuscripts. Many many ancient manuscripts from various languages such as Coptic, Gothic, etc etc. We have Lots of ancient Old Testament manuscritps, just look at the Dead Sea Scrolls! That would be my first approach I would do in order to give him soemthing to think about.