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The type of 12 gauge will definitely be a factor. Single shot, side by side, over under, etc, will create more felt recoil than a semi auto, or even a pump. The main issue is shoulder placement. I have seen, very small women/girls as well as boys, handle all of those with no issues. Low recoil, and mini shells, will alleviate felt recoil, but if this woman, plans to use her gun for SD, she will need to feel confident shooting it, with any load. Try to find an area, she can shoot without restrictions, on shot, even if it is private property, where she can get comfortable with lighter loads, and practice good habits, with shouldering and aiming, without the pain, and anticipation of recoil.

It may be that 12 gauge, or the type of shotgun, is not suitable for her. You can then reccomend, a different gun, which she could trade for.
It's great that you are willing to help out someone who was obviously not properly trained, and now you have a great opportunity, to show, that it's never too late to learn.