Is not so much how often you use those keys, but whether they are available or not. The great thing about controllers is that you can transpose the keys, so that if you want to play something an octave or two higher/lower, you just press a button and the notes are automatically available.
As for features, it depends on what he wants to do with it. If he is looking to use it as a piano/keyboard, the M-audio works perfectly fine. As I said on my first post, you can't go wrong with that one. But if he is looking to experiment with sounds and effects, there are better controllers out there. Honestly, and not to try to take the "easy way out", but it might be good to just take him to a music store and have him try out an instrument or two. If there is a Guitar Center or Sam Ash near you, I would pay them visit.