AR in 5.56 and a drop-in .22 kit.
+1
.22 ammo is cheap and not intimidating. .223 isn't that rough either, but it's definitely enough for a SD/HD situation.
12 gauge recoil can be quite intimidating. Why would you give a complete neophyte a 12 gauge unless you're trying to make them think twice about firearms? Take someone who's never shot before, buy them an 870 and take them to a range to shoot some birdshot and 3 1/2 mag rounds ? Not if you want to remain their friend...
If this is something that's not going to be practiced, or used , and kept under a bed, etc. Then there are a few options:
A).223 AR with frangible ammo
B) 9mm handgun/hi-point with frangible ammo (hi-points might not be top of the heap, but a rifle-format firearm can be more intimidating than a handgun)
(if this is truly a HD firearm, frangible bullets will mitigate the risk of taking out friendlies 2 rooms away from the action.
C) 20 ga. auto-loader with birdshot
-Working a shotgun slide is not natural, and could easily stump someone with little experience in a high stress/life or death scenairo. An auto-loader removes this risk. And you don't need 00 buck or a slug to shoot someone across a room. Smaller shot will also reduce the chance of downrange collateral damage.
D) 12 ga auto-loader with birdshot.
E) Taurus Judge
-Intuitive point and shoot and 410 or 45 shells. Good for CQ situations.