My Wife loves my SA XDm 3.8 9mm. The standard XD can be had for less. However, I got my XDm for $479.00. Add the shipping and transfer, it came to around $520.
Personally I'm not a big fan of 9mm or .40 S&W, but I really like the XDm 9mm. I have big hands, and the grip is excellent IMO. My wife has fairly small hands, and she likes the grip as well.
Simply put, the heavier the pistol the lower the felt recoil. This will vary slightly based on grip design and the barrel centerline in relation to grip position.
I understand that the S&W M&P seems to have low felt recoil. I've never shot one so don't take my word for it.
If possible find several different models to rent or borrow. Let her try 'em all. Low recoil won't make her happy if she hates the way the gun feels, or even looks. In other words, if she don't like it, she won't want to shoot it.