I did my centers the same as my background. I wanted it to look like an antique on I had seen with no appliqued center piece.
If it were me, I'd take her shopping if possible and at least but a few pieces in colors she chooses. You can take those pieces and do the big shopping yourself. If she can't shop with you, send her a link to an online store you like. Have her put some items she likes in the "cart" and send the link from the cart back to you. That way you will have a really good idea of what she likes to work from.