Can't speak specifically to your wife's build, strength, or general abilities, but most females (young and old) can be shown how to rack and lock back any normal slide.
Is she using an overhand grip, getting all four fingers of her off/weak hand on one side of the slide and the full heel of her weak hand on the opposing side? Is she holding the firearm close to her? Is she (mostly) pushing forward on the frame while pulling back on the slide? To lock the slide back, does she slightly rotate the grip in her hand if neccessary, pre-positioning her thumb below the slide stop and pushing up with her thumb while simultaneously racking the slide?
And importantly during all of this is she able to keep the muzzle pointed in a safe direction at all times and not sweep her forearm or anything/anyone else?