"Never shoot a large caliber man with a small caliber bullet!"
But guns are personal, and you want something she can shoot accurately (shot placement is key!), carry comfortably (hopefully not in her purse), and isn't afraid of!
I recommend finding a range that rents handguns and having her try a few. Some places have programs specifically tailored to new and/or women shooters.
Or go to a gun show and have her handle a few handguns to see which fit her grip and of course which ones she likes.
Or find a few buds with handguns willing to bring them to the range so she can try them.
I taught my nephew's girlfriend how to shoot. I started off with a GSG 1911 in .22, then moved up to a 1911 in .45ACP.
Lastly, but far most importantly, make sure she is properly trained on how to handle a handgun safely!