As far as handguns go, the larger the frame, the less felt recoil. For a small woman, 9mm would be the easiest round to shoot thats still going to be enough to stop someone, so long as its filled with good home defense rounds. Handguns are also the easiest walk around with, shotguns tend to be heavy and hard to handle is close quarters.
There are many good handguns out there, some that I would suggest (and all in full frame size):
Glock
H&K USP
Springfield XD (I have one and love it)
Hi-Power
Smith & Wesson MP series
Ruger P Series
Sig
Walther P99
My personal favorite is the 1911 platform. The cheapest one you are going to find in your average gun shop would probably be the Springfield GI. These are going to be chambered in .45ACP, but the recoil honestly isn't all that bad. .40 has worse recoil than 9mm or .45ACP. If she really wants a smaller framed gun, look for something in .380, which is a shorter version of 9mm. A very good example of this is the Bersa Thunder.
There's really a wide variety of good guns out there, but expect to spend no less than $400 on a quality gun, although there can be exceptions to that.