Yeah, once you add some sort of pairing process, you can get most of what you want -- though your users will soon find that what they gain in better signal from more APs they probably lose in WDS overhead, especially once you go up to 3 or more of them. Probably not noticable if they're just browsing the web and downloading a file or two, but if Dad's watching Netflix, Mom's downloading iTunes, and junior is gaming, it might be an issue.

Any response to my question about why your friend can't just move the existing router (and whatever modem is connected to it) upstairs? Even if he has to run some cable, he would avoid spending money on another box, and keep the simple network topology.

Re: the crappy software quality on home routers, it's always surprised me that one of the big vendors hasn't worked out an arrangement to ship with a DD-WRT or Tomato fork. Imagine if Cisco or D-Link hired or contracted one of the big open firmware gurus to develop a more consumer-friendly Tomato or DD-WRT build for them. Heck, TomatoUSB is mature and stable enough right now that you could hide a bunch of the glitzy features behind an "advanced mode" checkbox and ship it as the factory firmware on nearly any router out there. Why they would insist on sticking with their generally crappy firmware that people end up replacing anyway is beyond me.