Its just a less cluttered look than the usual country decor.
Not so feminine and ruffled looking.
Not the cutesy look.
Its clean cut with a country touch instead of all modern.

Modern Country seem likes an oxymoron, like jumbo shrimp or government intelligence. I don't think you can have both - you have to pick between country and modern.

IMHO it would be a good idea if you got clarification from your customer.
There are so many very knowledgeable people on this board and most are not sure. My quess would be what Rose Marie said, but I sure wouldn't start a quilt without knowing what the customer actually expected.

Please post a picture when it's done! Good luck!

While I think you should ask your customer a few clarifying questions, I can't resist guessing at what she wants :-)

I'm thinking she wants a traditional pattern using the colors she specified - but making the quilt out of relatively plain, single-color fabrics, e.g. one-color batik, tone-on-tone, or even solid color fabrics.

JMHO, of course. I'll be eager to learn what she tells you!

I would show her a few patterns and let her choose one or have her show you a picture of what she has in mind. This summer I was asked to make a quilt for an upcomming wedding. The clint started to decribe what they wanted but I was still unclear as to what they wanted so I asked them to go on line and look at quilts and email me a picture of something like they wanted. Worked out great. By the way, I would have gone in another direction from what they were telling me the picture cleared everything up.

Country: calico & gingham, pinewood furniture with heart cutout, handmade baskets, faded looking, 1930s look, muted tones and neutrals

Modern: sleek, shiny, hard surfaces, sharp lines, Black and White, monotone, crisp colors, saturated brights.

These are some typical looks. Make a list of your own, as a point of departure for your dialogue with your client. Better yet, show her some quilting books, magazines and websites where quilts are shown. Let her point and choose.

Get a copy of the book: this description gives some excellent clues to use: Modern Country
by Nancy Ingram blends the quiet, simple elegance of classic country with a clean, modernist edge to create an entirely new interior genre. When classic country is infused with modern interior design, it becomes bolder. It offers a contemporary attitude, a confident feel that mixes periods, patterns, and pedigrees but embraces the element of restraint. As noted photo stylist and author Nancy Ingram says, "Mix it up and pare it down!"

Modern Country interiors are fresh and lively, with color palettes that include the classics, but combined in new ways or intensities. Monochromatic themes-restful, quiet, calming hues-come into their own in Modern Country. There are no rules-only creative ideas and the courage to blend new with old, traditional with modern, to forge a look that is inviting, warm and even whimsical.

The hue of the colors she named don't give you much information. The value and tone or shade would tell you a lot. Burgandy, deep blue, deep green say jewel tones to me not rustic colors. It's all in the value and tone or shade. You need a LOT more clarification. Beside having her give you pictures of quilt patterns that she likes, you should have her give you paint chips of the colors she actually wants. Is she wanting the clear pure hues, hue (color)
tones with white or shaded with black? Unless you have a really good color wheel that you can show and discuss with her, she needs to go to a paint store and pick out the colors she likes. Then does she want these colors in patterns? Small patterns? Large patterns? floral? paisleys? You have NO workable information at all! Good Luck.

Stop in a furniture or department store and ask to see some new stuff in Modern Country.

If she has any family pictures taken at her MIL's home, she could show you those to help get a feel for this woman's style.
Your pre-planning dilemma is just another reason special order quilts should cost $$$$$$$$$$$$$$$

Along with all of the other great suggestions, I would suggest finding out if she is a fan of the Pottery Barn catalog. I consider my home decorated in Modern/Country or Contemporary/Country and most of my inspiration has come from there...