The way I see it, and I agree with some points already posted, is that a woman can carry and shoot any handgun she's of a mind to just like a man can. Sheriff Jim Wilson once wrote that 80% of the shooting population can shoot a 1911A1 and he included petite women. My sister prefers an M85 Taurus .38spl and she can shoot any of my guns she wants to, but that's not often because she doesn't tend to like my choices. On the other hand, I can't hit the side of a barn from the inside with her's. In my experience, the grip's size and shape has a big effect on trigger pull and how that effects the sight alignment. Little skinny grips seem to me like they make the trigger feel heavier. The shorter the sight radius, the better the operator's trigger control has to be. (Read some Ed McGivern on that.) So, you can take any given double-action revolver and change things like the grips and trigger pull weight (by changing spring kits) and she'll think it's a totally different weapon.
I agree though that she needs to choose her own. She just needs to choose a quality piece and practice with it.
I mentioned Jim Wilson... I recall something else he said, and not that he's the only one that ever said something to this effect, but it's best to use light target loads to practice. Then shoot just a few rounds of the hot stuff to know where it hits when you need it.
This is not aimed at the OP because the OP already said his wife loves his 870. Just more thinking. And I've actually run into this. Regardless of which weapon she chooses, she'll have to practice with it. If she already likes to shoot, that's easier for her to get into. What I hate to hear is some woman walks into a gun shop wanting a little snub-nosed .357 to stick in her purse with full-power SD ammo and never practice with. "Dangerous Man Syndrome", the idea that a man with a gun is someone not to be trifled with, that ain't limitted to men. Someone with that idea is a lot more dangerous to themselves and bystanders than they'll ever be to a perp.