Yes, as Matt asked, the real question is if you can get signal from your roof easily enough. Do you have any sort of line of sight on the boosted AP? If you can't get signal on your roof, you'll need to invesitgate directional antennas.

Once you've devised a way to get the signal, what you need is something to relay the signal. One way to do this is to buy a repeater or get a wireless card with an antenna input, but that's not what I'd reccomend.

Instead, go buy yourself a AP Client and a good AP/router. (WRT54g is probably the best choice these days for access points). Plug your laptop into the ap client, get it set up and connected to his wireless network, and then plug that cable into the WAN port of your router, and configure it normally.

Now you've got a nice way to connect and your wired/wireless computers, and it should be fairly reliable and keep your computers secure from everyone else he's sharing with. You can investigate outdoor housings and power over ethernet if you need to move your AP client outside to get a signal.

Matthew