What I would do is get that silver ring for her to wear when she wants to wear just a wedding band…and a white gold band to wear when she wants to wear both her solitaire and her wedding band……many women (me included) have several bands that they switch out depending on where they are going and what they are doing….

..be careful on the silver rings, tho….that touch of gold may be vermeil, a thin coating of gold that always wears off in time….you can achieve the same look with bands that are both white & yellow gold.

As far as mixing metals, why not? Those who poo-poo that are too anal or metal/jewelry snobs…..I have two matching platinium & diamond eternity bands that I often will sandwich one of my solitaires between, even the yellow gold ones, looks sharp IMO.Most white gold today is rhodium plated so it doesn;t even look like real white gold but fine sterling silver-real white gold is a pale gray-silver, not the white mirror brightness natural to sterling silver. It won't even buff up to that shine typical of silver. So if she occasionally wears the solitaire with the sterling band there won't be a damage issue…and the rings will look fine together.