If he's trying to record guitar/voice using the mic. port on the Apple series powerbook/G4-5 series, it won't work. You can't get sufficient volume levels, and even the Apple gurus admit this.
Here's what I recommend: For an inexpensive USB interface, get the M-Audio Fast Track interface. Since there aren't many (only one I know of) MIDI guitars on the market, he'll be recording guitar tracks thru this microphone interface.
For MIDI keyboards, I'm using a very low cost Edirol USBtoMidi connector called the UM-1SX. Be sure - after you install the UM-LSX you select "Software Instrument" to record your midi tracks...and you're good to go. The Shure 8900 dynamic mic works great; provides sufficient sensitivity for guitar/vocals tracks, at a reasonable price too.