Ok, does his pda have integrated WiFi?
Does he have a pci or usb Wifi adaptor for his desktop?

If no to any of the above, then you need to buy a wireless adaptor for them so they can access the wireless network.

As far as limitations in adding an external antenna, most high gain antennas boost the signal so you're not really doing anything to limit yourself, but you need to make sure if its a directional antenna that you have it pointed line-of-sight with his building/house.