The main reason graphic design and A/V people use Mac's is that Apple got it's foot into those industries' doors very early in the game (whereas M$ focused on core business applications). Many claim that Mac's are better for these types of applications, but they're not. x86 architecture is just fine (Apple is switching to x86, afterall).
Digital audio and sequencing software involve pure number crunching of complex DSP algorithms, so you'll want a CPU with good floating point performance (Mac G4, Pentium M, or AMD Turion64/Athlon64), and you'll also want a lot of memory (at least 1GB) and possibly a spare external HDD, especially if this guy's going to be editing digital audio (not just using synths, sequencing, samples, MIDI, etc.).
Also, I'd hesitate buying a Powerbook or an iBook at this point. Since Apple is going to switch to x86 within the next year or so, and since it is unclear if many Mac software vendors will still support the current PowerPC architecture in the future, a Powerbook or an iBook probably wouldn't be the best investment.
Bottom line, if he's going to create his own mixes and/or sequences, samples, loops and/or edit digital audio (which is much easier and more versatile on a computer thereby opening up so many more creative opportunities), then he needs to research and choose his software package (ProTools, Cubase, Digital Performer, Logic, etc.) and decide on hardware accordingly.
