Did your friend really recommend Mere Christianity for best argument on his side?

Yeah. Then again that was probably "best" in the same sense that TGD was "best" for me: what came to mind on the spot. He's probably read a bunch of other books (and obviously, the Bible), MC was his answer to "which work is most likely to get an open-minded atheist to take one step in my direction".

In fact there were plenty of things in Mere Christianity that I agreed with, it just didn't make much of an impression on me as an argument for the existence of God. It boils down to "we have a strong intuition that there is an objective morality, therefore God".

(ETA: now that I think of it, though, he may have recommended the book also as an answer to the question "what is it exactly that you believe in". Looking back, there may have been some ambiguity to our agreement.)