I suppose any gun is better than no gun, so a 22 mag falls into that category for me.
For a legitimate defensive revolver, I would say .38 is the minimum with good ammo. I would prefer a .357 but that's just me. Smith and Wesson makes several J framed choices, I would suggest a bodyguard hammerless frame in 2" or 3", and I would prefer the stainless over the Aluminum or Scandium frames. The Model 640 fits the bill nicely. Get some good grips to fit her hand the best and then practice practice practice. The stainless steel's weight will damp the recoil down a bit and make it more pleasant to shoot.
Or if she prefers a lighter revolver, and you want to have a "Practice" gun and a carry piece then the aluminum frame 442 or even a 340 would be excellent as well...
At the end of the day the best one is what she is most comfortable I suppose.