If he wants a simple Protools rig to get started with, I would recommend a Mbox 2 Mini.
It's your cheapest option that will still give you a decent box...and another plus is that the Mbox is a good computer interface for playback from itunes or whatever you play music files on your mac.
An even cheaper option would be to pick up one of the original Mbox's off of Ebay. Since the Mbox 2's have come out, the Mbox 1's have dropped a lot in price...and are still very solid boxes.
I have a Mbox2 Mini, and it's pretty great, minus the powering issues I have with it and my powerbook...but since you guys would be using it with an imac, you should have no worries.
Hope that helps.