First of all, I enjoyed the book your friend will have you read. Ishmael is an interesting work that can most likely be categorized as fantasy, yet through the fantastical elements, it has a different viewpoint of western history than what we normally have. You should enjoy it. Also, the movie Instinct with Anthony Hopkins and Cuba Gooding Jr is loosely based from Ishmael. It should be noted that Daniel Quinn was not an author prior to Ishmael, and that he wrote the story to win a contest for environmental fiction sponsored by Ted Turner.
Mere Christianity is a very good book. It was the primary instigator for me to investigate Christianity when I was a senior in high school. I don't find very much of Lewis' writings contradicting Orthodox teachings, and from knowing dozens of Orthodox who credit Lewis with starting them on the path to the Church, I think this would be a safe bet. In fact, a priest friend of mine tells me that during his studies at SVS in the early 80s, a few of the seminarians created a Troparion for C.S. Lewis as a joke. As Linus said, it can be simplistic, so if your friend reads a lot of agnostic/atheistic arguments, this might not be the best place. If they have a fairly open mind, it would be a nice book, though.
A few others that come to mind...
Between Heaven and Hell by Peter Kreeft. This is a fictional debate between C.S. Lewis, JFK, and Aldous Huxley on the existence of God and Christianity as it relates to both secular humanism and pantheism. All three men died on November 22, 1963, so the book has them meet just after their death in a limbo where the debate must be carried out before progressing. The arguments here are a bit more developed than in Mere Christianity, but of a similar nature. I enjoy reading this book, but some may not.
Crime and Punishment. I think this a good idea, but not for the book exchange. Wait until the next time your friend asks for a novel reccomendation.
The Everlasting Man by G.K. Chesterton. I'm not an expert on this text, but seems like the sort of thing that would help. Others here can probably provide more info or read Amazon reviews.