I. PLATO IS A MUST!
Oh, you must choose Plato, he is the best! I say this because your friend is not necessarily religious (yet), and Plato is a good intro to philosophy. He is an extremely beautiful writer and is very easy to follow despite the depth of what is being discussed. Suggesting Plato would point to the direction of one of the three monotheistic faiths (in my opinion, Catholicism the most) without being obviously such an overt gesture like a book on St. Anselm, St. Augustine, and St. Aquinas would be (which could be handed when your friend has a good grasp on philosophy). There is a reason St. Severinus Boethius and St. Augustine hold Plato in such high regards. Seeing someone give accounts which correlate so well with Christianity in a time before it, by use of reason alone seems rather persuasive to me

. Anyhow…
II. Suggestion for the Book
The Republic stands out in particular as long as you can get past
some of the very early needless outlining of the city (which has some questionable morality, and even as Plato/Socrates suggests, an unnecessary in depth look at this imaginary society). If you get the Penguin Classic edition, I would just recommend your friend skip the part titled Women and Society (that is the useless area in my opinion). It is useless to the underlying philosophical argument and contains perhaps a suggestion of ‘infant exposure’. Whether Plato would endorse such a thing is questionable, especially in the way he mentions it there.
III. Further Suggestions
Besides that I cannot think of any real good intro to philosophy book, except maybe St. Boethius’s Consolation of Philosophy which is rather unique to say the least. As for a few tips from my experience thus far:
**1.)-**St. Anselm’s works are responsible for my love of philosophy and conclusively pushing me over the edge into Catholicism I think. Particularly his Cur Deus Homo (Why God Became Man) and his Proslogion. So obviously with that bias, I would suggest Cur Deus Homo as his first Catholic philosophy book. Anselm’s theory might not be perfect (and many dislike it), but I think it is a must to begin exploring the necessity of Christ’s ministry on Earth, and I find it very persuasive.
**2.)- **Aristotle and St. Aquinas I think should wait (just my opinion) until one has a basic grasp of philosophy from easier writers such as Plato.
**3.)-**Avoid the Early Moderns (‘Enlightenment’ people) if possible until one has read and understands a few of the Ancients and/or Medievals. Not only are some very boring and difficult to read (Spinoza and Kant stand out, even though I find reading them very valuable), but you’ll find the Ancients and Medievals solved the ol’ Moderns’ problems a long time ago.
Hope that helps! Good Luck.