A 91 isn't strong enough for a 22-250. I know that lots of small-ring actions have been converted to higher pressure chamberings, but it just isn't safe. You'll put the money into it to have it converted and your wife will be holding a potential grenade in front of her. SAAMI MAP for 22-250 is 65000 psi. Something to think about is converting it to the parent cartridge, 250 Savage, instead. The SAAMI MAP for it is 45000 CUP or about 50000 psi. Ammo availability is an issue, so it would become a mostly handloading affair. Perhaps that's a non-starter as well.
You might just be better off leaving the 91 as-is and getting her a newer gun. I've seen Savage Axis 22-250's for sale around here for <$400. Not the greatest rifle, but the vast majority of them are good shooters. And I'm not usually one to talk someone out of building on a mauser action. I've got 5 actions in various stages of buildage, including the aforementioned 1893 and a bunch of large ring actions.
Matt