ive been to an oasis in in Joshua Tree national park and i remember there being large filifera and robusta in the same wild habitat, most likely hybridizing.

i would say that your friends palms would definately benefit from a good soaking every month. or maybe twice just to help establish the root system required for it to reach that water table

Not sure about where to find Washingtonias in their native habitat (only know of the general region), but as far as watering goes, I definitely recommend giving it some extra water. Some light fertilizer wouldnt hurt either. I had a potted washingtonia that grew extremely slow and I think it had to do with a lack of nutrients and a lack of water. Older plants probably need very little additional water to do well, but they should get plenty of water during the hot summer months when young.
Good luck!
-Alex

I believe there are large native groves in Palm Canyon, some where near Palm Springs, CA. Saw lots of photos recently on another forum.

I'm resurrecting this ancient thread, because I think I can help with the first of the OP's questions: where can Washingtonia robusta be seen in its natural habitat? I've linked a paper which answers this very question. The short answer is that the most northerly natural populations are in the area of El Marmol and Catavina, Baja California, about 220 miles southeast of San Diego. I scaled off the distance from a map that shows the distribution of the four fan palm species on the Baja California peninsula (W. filifera, W. robusta, Brahea armata, and B. brandigeei). W. robusta also grows at other scattered sites further south, down to the tip of the B.C. peninsula. The authors used Google Earth to identify palm populations in so. Calif. and Baja Calif., supplemented with field work by others and their own survey from aircraft for areas that were not covered adequately by Google Earth. The authors identified more than 15 thousand palm populations, consisting of an estimated 1.3 million plants. Before someone sniffs that it's impossible to find palm groves in Google Earth, I suggest they download and study the paper and start their own searches in G.E. I found that it was quite easy to locate many groves, and it's possible to distinguish B. armata from Washingtonia spp. W. filifera and W. robusta cannot be distinguished from each other in Google Earth, but there's a gap between their natural ranges. Washingtonias south of the Sierra San Pedro Martir are assumed to be W. robusta. You can verify your new-found skills by studying known groves in California.

As one works with the paper and studies Google Earth, some interesting facts emerge. For one thing, although we tend to think of W. filifera as being rare and oh-so-precious in its few easily-visited natural oases in the U.S., it is in fact quite abundant. There are many plants in inaccessible canyons above the well-known sites, particularly in San Diego Co. In Baja California there are vastly more of them in canyons in the Sierra Juarez. The most famous oasis is Canon de Guadalupe, south of Mexicali, but there's a major drainage north of Canon de Guadalupe that has at least as many palms (hundreds, maybe thousands). Also, there are natural populations of W. filifera west of the mountain crest (Valle de las Palmas and Ojos Negros), and these are not very far south of the U.S. border. Another interesting fact is that Brahea armata comes within about 20 or 30 miles of the U.S. border. B. brandigeei is common in the southern mountains (e.g Sierra La Laguna), but for me those are harder to find than the other species.

Here is a link that might be useful: Palms in Baja California

Correction: Brahea brandegeei, not "brandigeei."