This happens a lot. Many women don't like crashing, or any sport that seems dangerous. Anyway, I wouldn't bother selling the bike. It's way overkill for gravel, yes of course, but it's fine to use it there.

I really like my Geax Saguaro for singletrack and gravel roads, but the Geax AKA is even better. Both of them roll really well and wear well. The Maxxis Crossmark doesn't wear as well and is heavier, so I wouldn't use it. I wouldn't put a CX tire on there because some gravel roads have spots with loose sand and that will get sketchy on narrow CX tires. If the bike were ridden by you, instead of your wife, on gravel then yes I'd use a 32-35mm file tread CX tire.

Then again, maybe I'm reading too much into this.