ok--first off, does she have any handgun expierence?
let her try different ones at the store and see which one feels good in her hands and what she would prefer to carry (weight, contours, etc) as well as hopefully like the trigger as well as the controls and slide (if auto)
some will say that a revolver would be the best to start women off w/ (reliable, no slide to rack, no controls to forget ), but i think thats kinda silly as if any guy can manipulate and learn how to use a firearm, then a woman who is willing to learn should be able too
i think that the Glock would be a great pisol to start off with--makes em learn how to keep the finger off the trigger till ready to shoot (4 rules of gun handling)
as to caliber, i would start with a 9mm and practice initially with a .22lr (if she is a noob); if she can handle the recoil and place consistant, accuate, fast shots with it, then go for it
if not, i would try the G19
h/w, i will have to say that the .40sw is a tad too snappy for me--i prefer the push of the .45
since this is a carry gun, the caliber (w/in reason) that is good and comfortable to shoot should be the one to pick to practice with all the time
the .40 is not a lighter recoiling round (imo) than the 45acp
btw 9mm--.45acp, they are all good manstoppers--some a tad better than others, but they will all work