Lamarca, I just looked in one of my computer books and found this for the iMac and MIDI keyboard:
You'll need, as usual, a serial-to-USB adapter, a MIDI interface...
an iPort expansion card
lets you plug in, among many other things, a MIDI keyboard. Get a MIDI interface and the music software of your choice. I bought the MIDI interface that I have at the same place I bought the keyboard last year. If she calls around to any music store that sells Yamaha keyboards in her area, they should be able to provide what she needs.