One thing often overlooked with women is whether they have the upper body strength to rack the slide in a pistol. So that's one thing to verify.
My wife liked the Springfield XD better than the xdm because the grip was smoother.
Your best bet would be to go to one of the indoor pistol ranges and rent several guns for her to try. Unless you know someone that has several you could try at the range.
Another gun my wife likes is my HK P30L.
If those are too big you'll need to look at a Sig P239. It's a single stack nine that should fit her hand.
Hope this helps.
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Why isn't every single response here a revolver? OP stated that this is a gun to learn on and use for defense if needed. It's a defense gun the very first day it comes home without her ever taking it to the range.
The possible need for home defense is every single day. The range days don't come everyday for learning.
This pistol needs to be simple and effective from the nightstand to the bedroom door, first and foremost.
It needs to be simple. Any quality 4"
.357 revolver.