I found the textbook "Genki II" very useful. It's not an official AP book, but it can help a lot since there is a startling amount of overlap between it and the AP curriculum. It has a green cover; you can't miss it. It's fairly cheap online if you get it used.
Your friend should also print out the official list of AP kanji; there are about 400 - 500; I don't remember the exact number. It's easy to find online. He won't have to write the kanji out on the test himself. It helps for memorizing, but all he'll have to do is recognize them.
One thing I'd caution him about is making sure he's good enough at listening and speaking. That's a skill that really can't be taught from a textbook. I find that the official Japanese SAT Subject Test audio practice modules you can find online are much more helpful than any official AP material I've seen regarding that part of the test.
I would not recommend having your friend get the typical "5 Steps for a 5." That book did not help me out at all, and it was quite expensive.
Oh, and tell your friend to download a flash card program called "Anki." It's free online, and it's really useful for helping its users learn vocabulary. One can learn vocabulary quickly with only about 10-15 minutes a day of work (though one can work more often if one wants). This time estimate does not include the time required to input vocabulary words into the program, which can be a lot if your friend isn't accustomed to typing in Japanese on his computer. By the way, the AP test will also require your friend to type on the computer and select the correct kanji. HOWEVER, if you've heard rumors that once you've typed something in, you can't backspace, they aren't true, though they may have been in the past.
Sorry if I've written you a novel here; I took the test this past year and both the test and studying for it were vivid experiences for me.