I'd go with sapphire in white gold. My wedding ring is a family heirloom from Paul's grandmother, but is platinum with sapphires on the sides, and I love it. It also does not have prongs that stick out and catch on things, which is great!

White gold won't tarnish, or wear down as quickly as sterling might, and it will look like the silver she prefers over yellow gold.

It will be her ring, special to her, who cares if "fashion" dictates a diamond solitaire?