
When I was dating my tall and slender 115# wife, I took her to a pistol match where she did me the extreme favor of picking up my 45 acp Colt Mark IV off the bench and posted a higher score than I did. After 35 years of marriage she continues to shoot well and handles a full house 357 magnum revolver almost - almost - as well as I do.
There is a difference between 9mm and 40 caliber recoil; if you do a lot of pistol shooting - especially rapid fire pistol shooting - this should become apparent by the increased difficulty and concentration required to, 'settle-in' the front sight between shots.
As certain pistol shooters will tell you, the endearing feature of 45 acp is that it's an easier caliber to control under recoil - In fact, it's the easiest caliber to control. This is because the bullets are big, heavy, and relatively slow moving. The recoil characteristic is more like a slow heavy push than the sharp fast crack you get from either a 9mm or, especially, a 40 S&W.
My suggestion would be for you to take your friend to a rental range and actually try a few different makes and calibers of pistols before you buy. It may end up coming down to how the pistol fits into the hand as well as how much the ammunition is going to cost in order to become proficient with that new gun.
For whatever it's worth, in my little corner of the world, more women seem to prefer the Walther P99QA over any other semiautomatic design.
Walther