The book to which you are referring is a work of fiction—and that needs to be kept in mind. To base your beliefs on something that is openly fictional is simply not wise or rational.
In other words, a book like this is only the product of an author’s imagination, and I know of no reputable Bible scholar or historian (Christian or non-Christian) who would agree with its claims about Jesus or take it seriously. There simply is no historical evidence to support its alleged “discoveries” about Jesus, nor is there any evidence at all that Christians have “covered up” the truth about Jesus.
If anything, your friend is the one covering up the truth by refusing to take seriously the only reliable records we have about Jesus: the books of the New Testament. As I read your letter I couldn’t help but think of Paul’s description of the last days before Christ’s return, when people “will turn their ears away from the truth and turn aside to myths” (2 Timothy 4:4).
Pray for your friend, that he will face his own spiritual need and turn to Jesus Christ. In addition, ask God to help you be a “living sermon” of Christ’s love to your friend, both by your words and by the way you live. Only Christ can bring us the forgiveness we need, and only He can change our hearts.