"Fit is one of the most important aspect of felt recoil. If it fits, it will be fun to shoot. If it slaps her in the face she won't have any fun"
"She was using one of my training guns, a Beretta 20, and I used masking tape and a dish towel (forgot the foam) to get her eye up and looking down the rib and then changed the shims for proper cast for a left handed Shooter. Once I got her fitted, she was amazing. And shocked because as she said" I can't even brush my teeth left handed". The key with that switch is to catch them before they have developed any muscle memory."
"The main thing is how it fits. If it fits them well they'll have a better chance of holding it tight against their shoulder. That one thing alone will help them absorb recoil well and not hurt them."
"We tried everyone we could get our hands on to shoot. Like the 6th one was a montefeltro. As soon as she shot it she said this is the one."
There is a pile of good advice here already. To which I add: try before you buy. In my own experience, shotgun fit is very individual, and no amount of someone else's opinion is going to make a shotgun fit your girlfriend. My go to is an old 12 ga Model 96 I cut the stock down on to fit me better and sanded the finish flat so It doesn't give me away with shine.
Anyone trying to shoot a gun with a stock that is too long will be stretched out, off balance, and will get the butt in the wrong place. Frankly that is gonna hurt if the gauge or caliber has recoil.
Let her try as many as you can find to handle. If she is small, try youth models. Don't buy a gun on the basis of "If she doesn't like this, then the gun can be mine." Buy it for her.