*Artemisia frigida--- Beautiful ground hugging native sagebrush, silver with delicate seed heads.
*Dalea greggii-----another silver groundcover, works well in zone 7
*Dalea purpureua--Purple Prairie Clover
*Blue Grama grass. It will work on a dry slope, clump forming, low and great summer seed heads.
*Nassella tenuissima--Mexican Feather Grass--fine, golden, glows in sunlight and very drought hardy, naturalizes very easily by seed.
*Muhlenbergia grasses--check out the varies types. Very drought hardy.
*Hairy golden aster. Tough native, low and you can walk on it. Naturalizes easily, yellow blooms all summer thru fall.
*Callirhoe involucrata--Purple winecups (perennial variety). Native. Spreads in a wide low mat with wine colored flowers.
*Santolina. Non native, drought hardy, there is a silver variety and a green.
*Lantana horrida. Will take the drought & heat, low wide spreading.
*Baileya multiradiata---Desert Marigold, blue foliage, very drought hardy.
*Berlandiera lyrata---Chocolate Flower
*Calylophus hartwegii--Fendler's Sundrops, low woody shrublike primrose
* Oenothera macrocarpa ---Missouri primrose
*Achillea millefolium or some other variety
*Melampodium leucanthum--Blackfoot Daisy
*Liatris punctata--Spotted Gayfeather
*Artemisia ludoviciana--dense growth, tough. Get the nearly white one.
*Salvia greggii--Small shrub
*Russian Sage--non native, can get large. Silver foliage.
*Ephedra

I am sure the grasses I listed will grow in that situation. I'm sure if I thought, I could come up with some others. I do well with all of these with no irrigation.

Ditch lilies :) Round up doesn't even kill them! (Orange wild daylilies)

TexasRanger has some good suggestions. Pink evening primrose might work as well too...especially in the spring.

See: 'Decision Theory' Start with that topic and start your research from there. if you work in any level of marketing and sales, you need a basic grounding in that and related topics.

Hint: Ask the customer about other plants and beds on the property and then you can offer a 'mix and match' of new plantings that will add 'value' to what is already planted on the property - design, textures, colors, etc. This will prevent you AND the customer from being confused with an over abundance of choices. You can reduce the number of choices by 'limiting' the parameters of the decision.

Texas Ranger and Lisa, Thank you!

Wbonesteel,
I live in WNY (Western New York State along the Niagara River,) the weather is much different here than in Oklahoma. If I were talking about a person with my climate, I would not have come here for help as I would have no hesitation in what to suggest as the best choices.
I came here for personal suggestions from gardeners (as they are the best) in the same vicinity to narrow the parameters so hopefully the customer will have high success rate with their final decision.
Remy