another vote for the DA revolver. .38caliiber or larger, medium frame or larger. 4" or longer barrel.
mechanically, it is about as idiot proof as it gets. MUCH simpler than any semi auto. Does better in long term uncared for storage, too. Loaded, all springs are fully at rest. No magazine to lose, or to not fully seat. Does ok with NO lubrication.
My Dad didn't touch any of his guns during the last decade of his life. My brother, who lived there, should have been keeping an eye on these things, but didn't.
Found Dad's Colt Govt Model in his dresser drawer. Dropped the mag (loaded with 7 rnds of ball ammo), racked the slide, and watched it move slooowly forward, stopping about a third of the way closed.
Savage 99 in closet, safety on. Frozen by oil turned to glue.
S&W model 28, same closet, perfectly and completely functional.
Anyone who has a gun for use "in gravest extreme" needs to know a few things. Call it training, call it instruction, call it what ever you want, but they need to know how to use the gun and when to use it, and when not to.
and, believe it or not, in this modern day and age, there are people who can learn these things without formal instruction. There are also people who see something on the Internet and are instant experts in all related fields.
If they aren't "gun people" but have some degree of interest "just in case" SIMPLE is the best advice for a gun choice. KNOW THE LAW is the best advice for general use. (and "know the law" doesn't mean what the news says the law allows, it means knowing what the law really covers, not a bunch of BS hype)