A variation of Occam’s Razor applies here; the simplest solution is often the best.
Mere Christianity is outstanding and serves as a good baseline for any other work to measure up against as it pertains to outlining what Christianity is, and why it is the Truth. There are few who are equal in this area than Clive Staples Lewis.
From a Catholic perspective, as others have mentioned Peter Kreeft is excellent, definitely in the top tier of Catholic writers and thinkers - not just within the present day, but my opinion in all time. He is definitely, especially if your friend enjoys philosophy.