If he is in town for a bit, you should send him west to Crownpoint or Gallup or the likes. You'd have better luck west.
In Albuquerque you can try Izzy Martin in Nob Hill (it's a men's clothing store, but the owner is an expert on Navajo textiles and, in fact, carries quite a few), or Andrews Pueblo Pottery or The Hanging Tree in Old Town. There are also a number of dealers in Santa Fe and Taos.
If you're going to travel to buy one, I'd go to Hubbell Trading Post in Ganado.
You should check out Palms Trading Co. at Lomas and 15th. They buy directly from native artists and offer some really incredible prices (at least on jewelry and pottery--I've never bought a rug there). I know they carry Navajo rugs, though I'm not sure how wide a selection they have. Definitely worth checking out, though, as they're right next to Old Town.
A couple of folks on Facebook suggested Wright's Indian Art on Louisiana north of Menaul. Also the Maxwell Museum of Anthropology at UNM.
Serious investment shopping, or just plain ol' tourist souvenir shopping? For the latter (and possibly the former? I don't know) he could check out Skip Maisel's downtown. We tend to take first time visitors there when they're looking for some reasonably priced turquoise jewelry or pottery.
I would guess that anything is Old Town proper is going to be relatively overpriced, no?
For the best quality and most certain provenance for good rugs, go to Wright's Indian Arts at 2677 Louisiana Blvd. I have been shopping at Wright's for 20 years and my mother for many years before that. Wright's has been in business for more than 100 years.