I highly recommend going to an Intamcy if you have once near you for a proper fitting. They will have all sizes available to try in, but only carry high end brands. But once you know how the brands fit you can shop around. I spent about 100 per bra on 2 bras, they were the best bras I ever had. I now buy the same kind online. I shop at bare necesities dot com. Prima Donna are the BEST, but I also like elomi. You might just need to buy a bunch online and then try them on and return what doesn't fit. My sizing is different in different brands.
With such a large cup and small band, she can't buy cheaply bras ever. Get some good ones. Her back will appreciate it.
Agree with this! I love Intamacy! Expensive bras but they do last if you take care of them.