A Mathematician's Apology (http://en.wikipedia.org/wiki/A_Mathematician%27s_Apology).
This is a classic text, and most mathematician's revere it. It contains a lot of very interesting insights about math and while Hardy isn't a philosopher he is writing about philosophical issues. If he hasn't read it yet, it's worth reading for the insights into mathematics alone. I studied math at undergrad, and it's one of my favorite books.
A little background: Hardy was a huge pacifist and so only worked on topics in math that he thought couldn't ever be used to assist in war. His main focus was group theory, which he happily commented was the most useless branch of all math (ironically, it has HUGE military applications in cryptography. IIRC some of his own results in fact. Sorry Hardy). He wrote it when he was old, but his main purpose in the book is to defend doing math on purely aesthetic grounds. He rejects the idea that math is worthwhile for its practical results (medicine, improvements in quality of living etc.) because it also has a lot of practical defects (improvements of mechanization also mean improvements in war: better killing machines, nukes, etc.). I think he also points out that those sorts of things (good and bad) aren't really produced by mathematicians anyway, and that it takes some time for a theory to gain practical utility, meaning that a mathematician often doesn't have that kind of thing in mind anyway.
From there he spends some time trying to argue what makes a mathematical result beautiful. He goes through some interesting and easy to understand examples. It's not very technical philosophically, but there are a lot of important issues that he is talking about in the philosophy of mathematics. If nothing else, I'm sure it's a book he'd appreciate.