Or she could try something that's not a Sig.
It's been awhile, but I have a hazy memory of Sigs with heavy slides. I distinctly remember their insane DA trigger, which, by some unfathomable reason happens to be unreachable for those of us with short fingers.
Recoil isn't just a question of caliber, it's also a question of the gun you're shooting it out of. Weight, internal mechanics and the fit of the weapon in your hands all makes a diffrence.
Example: I'm a .45 girl. I lurve me my 1911A1. I shoot +P ammunition out of that without any issues.
Glock 21 messes me up. Seriously, I get abrasions on my trigger finger from hitting the inside of the trigger gaurd, the base of my palm gets a nice Glock grip tecture pebbled into it and the streach between my thumb and index finger gets horrible bruised. The reason is I have to streatch my hand so much to grip it and reach the trigger, there's nothing to cushion the recoil.
If your wife is streaching out her hand to get a grip and reach the trigger, which she's likely doing even on the SA (it's not the length or grip on Sigs, it's the placement of the trigger) that's probably a bigger part of her recoil sensitivity then that "big" .40 S&W caliber.