How is she when she shoots your 9mm? Did she smile and want more and only complained about the size of the grip not fitting her hands? Has she shot at all so far? I'd base your choice on these factors before I worry about what gun to buy. If she has not shot with you at all can you can borrow a .22 from a shooting buddy first. Or have that buddy meet you at the range with his .22 semi or revolver for her to try? Better a .22 that she really enjoys than a 9mm she shoots just to keep you happy and think you're doing the right thing.
From helping out as an RO at a rental range I got to see a lot of girls dealing with different handguns. I saw that for smaller hands 1911 single stack guns fit well. And can be made even better with a shorter reach trigger. The M&P fitted with the smaller grip filler is another, The CZ in single action mode surprisingly works with small hands well too. And if fitted with the flatter metal grips is even better. And finally the Ruger SR-9 fits smaller hands well.
In terms of rimfire guns the Browning 1911-22 is an 80% size gun which fits small hands fantastically. The Ruger Bearcat is another small hand scaled down gun. But note that these two guns do not do so well in larger size hands. So either would most likely be "her" gun and if you shoot them you'll love the accuracy but hate how they fit your hand.
The smaller and slightly lighter K frame size revolvers are another good option for smaller hands. The ladies that have shot revolvers liked them both for the fit in their hands and also the fact that they don't toss brass all over and down places they really don't want hot brass to go. And this might be a good way for you to get a revolver that she lets you use now and then. Note that the Alfa Project revolvers are virtually the same for size and weight as the S&W K frame revolvers. So either would be a good option for your girl friend if she turns out to be a wheelgun girl. And speedloaders that fit the S&W K frame guns also work with the AP guns. K frame examples are Models 10, 14, 15, 19, 66, and some others. But those are the ones that leap to mind at the moment.
Needless to say you NEED to take her and the suggestions and go shopping and let HER find the one that fits. I'd no sooner buy a handgun for a girl sight unseen than I'd buy shoes for her. Besides, that sort of shopping trip is a win-win for a date...
On the other hand does it need to be a handgun? A .22 or 9mm rifle might be an option she likes and which you can enjoy as well.