I think its all dependent on your wife bud. If shes commited to it, then she should be fine. My ex-wife was 5'2", 103lbs and the only real meat on her were her god given recoil pads. She shot my 870 super mag w/ 3 1/2" turkey loads w/o too many issues.It'd bruise her a little, but it was the only negative, and was quite minor.
I sell a good deal of security gauge shot guns assuming she is using a 2 3/4 field load (7-8 shot) and simply aiming for center mass it shouldn't be too bad security grade SG's are designed to be used if you are terrified, and the adrenaline is pumping so the aim doesn't have to be dead on one of the many reasons I tell people who insist on a .22 for home security to piss off. That being said it is advisable to take her to the range to get that comfort level up also some people prefer smaller sizes (410, 20Ga) at that range it isn't going to make a massive difference. Just make sure the gun is sized right for her and don't get an auto, anyone who sells an auto for home defense should be reported to the BBB.