In my experience, many women instinctively try to manipulate the slide as a fine motor movement using the smaller, weaker muscles of their fingers, hands, and wrists and thus have difficulty racking the slide. As has been pointed out, however, once shown how to make the task into a gross motor movement by grasping the slide with their whole hand, locking their wrists and elbows, and using the larger, stronger muscles of their shoulders and backs to rack the slide, many women are surprised at how easily they can accomplish what previously seemed difficult or impossible.
That being said, if your lady still has trouble with the slide or simply wants a revolver, there's nothing wrong with that as revolvers can make excellent self-defense firearms as well. As to what, specifically, to look for, that depends on a few different factors. The first question that you need to ask is whether your fiancee will carry the gun concealed or simply keep it for home defense.
If the answer is the latter, that actually simplifies things a bit. For a home defense gun, there's very little reason to go particularly small or light. Medium and large frame revolvers with longer barrels generally recoil less, have better triggers, are easier to shoot accurately, and may offer more rounds. A medium or large frame revolver will usually have a better trigger because their hammers are often larger and heavier than those of small frame revolvers and thus do not require as heavy a spring to reliably detonate primers. The extra weight of these larger guns (particularly those with all-steel construction) dampens recoil considerably and the larger grips allow for a fuller, firmer, more secure, and more comfortable hold. Finally, longer barrels give you a longer sight radius which makes accurate shooting easier and also usually give somewhat higher velocity. That being said, too long a barrel probably isn't advisable as the gun may become cumbersome and difficult to maneuver in confined spaces and give more leverage for an attacker to wrench the gun from her hands in a struggle. IMHO, a 3-5" barrel is ideal for a home defense revolver with 6-6 1/2" being the maximum practical length.
If your fiancee plans to carry the gun concealed, that add another wrinkle to things. While medium and large frame revolvers are generally easier and more pleasant to shoot, they are also more cumbersome and inconvenient to carry due to their weight and bulk. What you must do at this point is find a revolver that offers an acceptable balance between ease and comfort of shooting and practicality and convenience of carry. The manner in which she chooses to carry will play a large role in what type of revolver is best. For example, carrying in a thigh or pocket holster requires a smaller, lighter gun than carrying in a belt holster or handbag. Just bear in mind that, for a given cartridge, the lighter the gun the harsher the recoil will be and the shorter the barrel, the more difficult it will be to shoot accurately.
Regardless of whether she plans to carry the gun or not, there are some universal concerns with choosing a revolver. First and foremost, you must ensure that she can manipulate the double action trigger without difficulty. This is of particular concern because many people who have trouble racking the slide of a semi-auto also have difficulty dealing with the double action trigger of a revolver. Secondly, ensure that the grips fit her hands well as the wrong grips can make an otherwise pleasant-shooting revolver downright painful and vice-versa. You want grips large enough for her to hold the gun firmly and securely but small enough that she isn't forced to take an uncomfortable or awkward grip in order to manipulate the trigger. Also, if the grips have finger grooves, make sure that they fit the size and shape of her hands in that she doesn't have to unnaturally spread her fingers apart nor squeeze them together. Finally, choose the most powerful cartridge that she can shoot comfortably. While a full-power .357 Magnum is a formidable defensive cartridge, it's recoil is usually too heavy for inexperienced shooters. Likewise, while a .22 Long Rifle is easy and pleasant to shoot, it's low level of power makes it a poor choice for those that can handle more recoil.
Insofar as specific recommendations go, it's difficult to do that without knowing a great deal of very personal information about your fiancee (much of it not the type of information that I would encourage you to share on the internet). However, I can recommend a couple of guns that seem to work well of large numbers of people.
I've yet to meet anyone who could not shoot a S&W K-Frame at least reasonably well. These guns have excellent triggers (some of the best available IMHO), are heavy enough to dampen recoil well without feeling clunky or cumbersome, and have a grip size which seems to fit a very broad cross-section of hand shapes and sizes. S&W K-Frames have been made in a variety of barrel length and come in several calibers the most common of which include .22 Long Rifle, .38 Special, and .357 Magnum.
If you need something smaller and lighter, the Ruger LCR is quite popular. While a small-frame lightweight gun, the LCR has an excellent trigger for a gun of that type and many come with the superb Hogue Tamer grip which does a great job of absorbing recoil. The LCR is available in .22 Long Rifle, .38 Special, and .357 Magnum with a 1 7/8" barrel as the only available length.
As far as caliber is concerned, that's another choice we can't make for you. However, .38 Special is a good place to start as, with careful load selection, it can deliver impressive terminal performance while, depending on the revolver, still producing only moderate recoil. Bear in mind as well that revolver chambered for .357 Magnum can also fire .38 Special and .38 Special +P ammunition and thus give you a great deal more versatility than a revolver chambered strictly for .38 Special.
