A slug from a 12 gauge should be plenty enough power to stop a black bear at close range. Of course, some slugs are much better than others. The shooter should have aimed at the neck, however.
Center of mass for the first shot - I would go high chest, I think, but not neck. Too easy to miss when the panic's on.
Followup shots to the neck and head, however, would be a really good idea.
I have to disagree about 12 ga. when the chips are down. I want a nice big Nosler with serious muzzle energy behind it, and I want a bunch of them in that bear in quick succession.