Truly the cartridge matters far less than the way the gun fits her, and it's weight. A rifle that doesn't fit her well, is to light, or has a short barrel, will all magnify recoil to the shooter.
The short barrel is more of a perception thing but it's real for shooters with less experience. Likewise, fitting the stock to her, and the gun being heavy enough to be comfortable with out being unwieldy is key. I'd recommend a barrel no shorter than 22".
My wife has been my learning experience, and she now uses a .257 Roberts most, and a .270W during elk season. Length of pull for these are short for everyone else in our house. I actually bought a .308 with a short barrel and fairly light thinking it would be good for her, she hates it.
