I think he wants the simple answer.
It's easy, just install a ventilation fan in a convenient closet and put a GFCI outlet in on its own circuit. Simply put a computer in there running Linux w/ snort & a good firewall setup and configure it as a DHCP server. A good old fashioned Xeon will do good as your router, and any old LCD monitor will be fine for controlling it when you can't administer it remotely. BTW, I suggest you use standard rack mounts for everything. A 1U server will fit fine. Then, you just put two network cards in, one to your cable modem/dsl and the other to a a Cisco 8 port 10/100 switch (also 1U, easy to screw into the rack). All you need to do then is just punch a couple small holes in the wall around your house, thread some Cat-6 cable (or better yet, run conduit, THEN put in the Cat-6, that way you're planning for the future) through the walls to the holes and install some RJ-45 female jacks. At that point, it's childs play for your friend to hook her mac up to the network.
Easy as cake.