Based on the measurements you gave, I think you might want to give the Silverado a try. The "ample figure" comments about using the Dore really are for the well endowed, but not overly plump figures. Because your friend is a fuller figure through the waist, she might benefit from the defined bust line. We really have to throw out the under bust measure as it is smaller than her waist. The corset will form a straight line from waist to bust, and in her case, that is only about 6" difference. She might look like a tube in the Dore. Hopefully, along with being plump, she is squishable too. Then you will be able to get some nice waist reduction. If she does have nice squish, then plan on a waist reduction of about 4-6". If she is a solid figure, then stick with the 2-4" reduction. Also, for her size, a wider gap would be better. So up to about 6" would be fine. Which will also give her some leeway for finding out how her body adjusts to a corset.
Ok, so for her measures, my plan of attack would be to use the C cup sizing, and then scale out 2 extra sizes. If she is squishable. For a firm shape, I would use the B cup sizing instead, same 2 extra sizes bigger.
You know, I just realized.... Laughing Moon now offers this corset in larger sizing in a separate pattern add-on. You can find out more about it at her web site: http://www.lafnmoon.com/