You are an extremely kind person to even consider taking this on. There are difficult fabrics that don't do much for each other, with a landscape design that usually depends on fabric selection for success. (Personally, I find the overall concept somewhat bizarre, too.)
Could you get them to settle on something else, like several square, decorative pillows? That way, you could group the fabrics in the best way, the design possibilities would be way easier than making a picture, there'd be more than one of them to be shared, and they could also be displayed instead of used.
hugs, Charlotte