As a hoofcare practitioner, I can say catagorically that frogs most certainly do callous. I wonder how your MIL thinks all those poor wild horses cope!
If they are regularly trimmed by a farrier, or stay damp &/or thrushy, they aren't likely to callous well tho. She may be right also in pointing to frogs as part of the problem tho, as recent studies have shown that it's more the sensitivity of the frog/heel than the sole that causes grief on rough ground. But I wonder how she(& many others) thinks a rim of steel only protecting the hoof wall helps that?
Conditioning is not the only requirement for a barefoot horse to travel comfortably. She needs to have a *good* regular trim - at least 6 weekly. She needs lots of exercise. She needs a good but not rich diet and ideally she needs to live on dry, hard ground. The softer & wetter her paddock, the more you will need to condition her to hard or rough ground. The occasional walk will likely do little and if she's on soft pasture you'll probably need to do at least 3 good walks per week to see much improvement. The more often the better.
If you don't want to shoe her and you can't afford the time or find a more suitable environment for her, or while you're building her up to coping bare, there's always hoof boots. Quite a few good brands available these days. Generally horses only need them on their front feet and you can take them off when not needed.