A good semi-auto action shotgun (gas operated and not spring operated) will have far less recoil than any O/U.
Keep that in mind.
And start with light trap loads and progress to duck loads.
For someone who has never fired a shotgun it is far better to start slow using light loads and the softest recoil you can find. You'll be farther ahead
As Semi-sane posted a 20 is a good gauge to start new shooters with.
Find a buddy and borrow a gun or two to see if she likes it. Then maybe purchase.
JW