Maccaby's, "The Mythmaker: Paul and the Creation of Christianity."
Spong's: "Liberating the Gospels", "Resurrection: Myth or Reality?", "Rescuing the Bible from Fundamentalism" and some other works.
I will get my paws on at least Maccaby's work soon enough, but if anyone has any insights for things to look for to counter the arguments they make, and then convey these ideas to my friend, I'd be happy to hear them.
Cross Posted to catholicism