Small, light guns recoil like a mule kicking you in the face.
My recommendation would be a 12 gauge just for the sheer lethality, and then a mercury recoil reducer on the mag cap if they are going with a collapsible stock. The mercury recoil reducer in my Benelli SBE and Beretta 391 Teknys butt stocks seriously tame the recoil compared to my o/u shotguns.
I would also suggest a semi-auto. I know I shoot a semi-auto and o/u way better than the Browning BPS that I hunted with for a decade. The difference on the skeet field was extremely noticeable. No need for he or his wife to have to add an additional step to the mix (i.e., pumping the shotgun).
In life or death situations recoil is about the last thing on ones mind. Have you ever really "felt" the recoil when killing a deer? If Mr. Intruder was entering the bedroom or wherever recoil is the last thing on ones mind. A semi in 12 ga would certainly be fine as well.... short barrel, I think my Browning B-80 had a 26 inch tube, that would be a good choice too. I picked the SXS coach for maximum simplicity for the OP's situation and expertise involved. As much as I like a 870 that would be further down the list for the green pea.