12ga Stoger coach gun(home defense version)...
Of the 2 common formats; auto vs pump action, I'd lean towards the pump action. A few top US instructors like Massad Ayoob, Louis Ayelbuck(check spelling) & John Farnam have guides and videos for defense training with pump shotguns. A couple(man & woman) could learn the basic drills with a well made pump action. The semi auto style seems better suited for SWAT or estate security type functions.
If a woman(wife-girlfriend) can't or won't learn to use a full scale 500 or 870 model pump, maybe a simple 12ga sideXside or O/U defense shotgun could work. Stoger puts out a "tactical" type with 1913 rails on it for lasers/white lights.
Someone could train to operate a coach gun even under stress. A white light would be a smart add-on to prevent ADs or mistaken IDs.
New low recoil LE/defense shotgun rounds are out there too.