I think if she knows how to use a shotgun well, something like the Mossberg SA-20 (mentioned already) might be a good choice. Fortunately, I've never had to actually use a shotgun in a defensive role (though there was one time I sincerely wished I had one)...BUT...I can imagine if I had to, the desire for maximum firepower might trump my concerns about recoil. Like most hunters I've met, you barely seem to notice noise/recoil when "in the moment"...I'd have to wonder if you'd feel the same in this role? No idea.
If the gun isn't physically too heavy, a 12ga Remington Versamax is one of the lightest-recoiling shotguns I've ever used, including single-shot .410, 16ga pump, 20ga pump/semi, 12ga pump, etc. It's my 15 year old son's favourite gun to shoot. There is a new version (R12?) coming out that is shorter/tactical, I don't know whether or not it's coming to Canada..I think so? My experience with any other modern 12ga semi is just about zero...but this 12ga Versamax I have chugs along like a sewing machine. Super easy to shoot, but it's weight probably plays into the very light, felt recoil.