Standard advice is to go to a range that rents guns and have her try out several.
Just don't buy her the wrong gun. Not only will she then be pissed, but she'll be armed too!
Of course, you can also buy a gun for you but pretend it's for her. You'll give it to her and she'll whine about it but then you can keep it. This works most of the time for me but I don't think she believed me when I brought home the S&W 500.
;-)
That was my plan. Buy the Ruger KP90 and have her whine that its too big and she doesn't like the .45. Then I get to look like Magnum PI while I 'm blazing away wih my new .45 Auto!