Go with the Easy Piano version. Big note books are generally for absolute beginners and are even simpler than the Easy Piano versions, at least in my experience. After 5 years of playing, your cousin should easily be able to play through the Easy Piano book, unless, I suppose, he's significantly younger than 10 years old, in which case he might not be able to.
What do the books have in them? Do you know what sort of things your cousin is playing? You might want to do a small bit of research and find out if he's really on beginner's level or if he could handle "real" pieces of music (not easy arrangements), such as Bach's 2-part Inventions, Clementi's Sonatinas, Kuhlau's Sonatinas, etc. Those are all considered "easy enough" to pianists, particularly after 5 years of lessons; chances are, if he's been doing classical piano, he's already played some of those....and they come as written by the composer, not watered down for beginners.
If he's older and even more advanced, you should *also* do a little more research and find out if he's interested in classical piano or something else. I can't really help you out with the "something else" but you could consider Mozart sonatas, Chopin preludes, Beethoven sonatas, and, well....it could go on for a while. So, is there any way you could find out a little more? That might help us out.
Otherwise, I'd say go with the Easy Piano version.
Hope that helps...
Topic: NEED DESPERATE HELP