I've already mentioned that the unreasonableness of Christianity is what she cites as the reason she has fallen away. Her words: "You can't prove Christianity by reason. It always comes down to faith."
The irony about that is that if Christianity were provable by reason, we wouldn't need faith--but, indeed, faith is what God asks for.
And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him.
Also, Jesus' words come to mind,
"...you have hidden these things from the wise and learned, and revealed them to little children. Yes, Father, for this was your good pleasure."This is part of what doesn't make sense in Christianity, part of why God asks for faith and not for understanding. If it were understanding that he wanted, we could earn heaven by just learning enough and deducing enough. No, somehow faith is what he wants. But if you actually don't believe--because of problems with reasonableness or problems of suffering or disappointment with God or, or, or--then how can one get faith?
(Personally, I believe, but I can sure relate to that guy in Mark who said, "I believe, but help my unbelief.")
When she explained to me about her loss of faith, she told me, "There is a saying: 'For those who believe, no explanation is necessary; for those who do not believe, no explanation will suffice.'" The funny thing about that is when I researched this saying, I learned it is attributed to a magician, or mentalist, called Joseph Dunninger. No Christian there.