lol, that's an interesting question:
It depends on the controller: the on-board controllers, for example the Promise Fastrak Lite, allows only for 2 drives. If you have a the full fledged PCI controller, like the Promise Fasttrak 100 or the new TX, you can hook on 4 drives.
If your friend has an on-board controller: it's 3 IDE HDD devices on total
Raid: 2 drives
Direct: 2 drives (master and slave)
If your friend has a full PCI controller: it's 6 drives in total
Direct: 2 drives (master and slave)
Raid: 4 Drives (2 master + 2 slaves)
but wait: could be even more, you could also use the connector for the CD, even though that will run only in ATA 33 mode.