+1 on what MikeC says. I've been carrying on and off duty for over 25 years. I can carry almost any gun I want on or off. On duty I carry a .45 but off duty it's a little .380 Sig P230 or a Smith and Wesson model 36 in .38. For off duty or civilian CCW, if it is big and/or heavy, there will be times when the owner won't feel like carrying it. A light and small gun even with a modest round is better than a cannon left at home.
If I work an off-duty gig or travel in parts unknown I carry a little more firepower (H&K P2000sk) but even that gun is only a 9mm. We drum into our recruits who are eager to buy the biggest, baddest, slickest gun they can for their first off-duty gun:
1. Pick a gun that will likely always work when you pull the trigger, that means a revolver and maybe a Glock.
2. Pick a gun that will work if you have to pass it to someone else who may not be as familiar with it as you...something that will allow them to simply pull the trigger and shoot. Revolver and maybe a Glock.
3. Pick a gun that will not make you look like a bloodthirsty fiend looking for his first kill when you sit in court. A moderate caliber (.38 or .380/9mm) to medium (.45-357). Once again, a revolver and maybe a Glock.
4. Pick a gun that will not tend to accidentally fire while being bounced around or placed /removed into/from a purse, fanny pack, man purse, holster, etc. A revolver, and maybe (less so) a Glock. With the Glock, be SURE to have the heavier trigger installed!!!
Remember, in a SHTF situation even cops panic and forget to release safeties, etc. so you want something that will not require alot of muscle memory training and can be brought to bear in a total chaotic, noisy, dangerous situation when your brain is on overload. A revolver and maybe a Glock.
Cops train all the time and we carry almost all waking hours. I can get away with packing semis, revolvers, "safe-actions" (aka Glock), etc. but a regular civilian generally practices once in a while so the KISS principles apply. For me, I'd get her one of the lightweight Smith and Wesson 5 shot .38 cal. revolvers. Small, (some models) hammer free so no snagging on draw, reliable as a hammer, doesn't impart a Dirty Harry or Rambo persona, affordable, and anyone with fingers can operate it without prior instruction.