There are 3 lights connected, 2 in one room and the third at the bottom of the stairs. There is a 3-way at the top of the stairs and another in the room with the 2 lights. This guy would like to also be able to switch off just the 3rd light at the bottom of the stairs. I did not dig into this too far but it appears the load, 3 lights and 2 switches are tied into one box. I can't throw in a single-pole before the 3rd light because he would like to turn it on/off from the other 2 locations as well.

One solution is to use X-10 devices, which will allow a remote switch (transmitter) to be located wherever desired along with a receiver relay at the light desired to be controlled separately.

http://www.leviton.com/sections/prodinfo/newprod/npleadin.htm

Review the information at the link I have provided and get back with any additional questions. There may be comprimises that may be necessary, but this may work out for the situation.

I checked out that page and copied it for the guy to have a look at.
Hopefully this will work for him :)