I have to recommend a 2"-4" 38+P revolver with lasergrips for a lady that is petite and not a 'gun person' or regular shooter. If she isn't trained well, she'll instinctively focus on the threat instead of the sights. That's why people miss, and lasergrips can help greatly. Get an all steel frame to keep the weight upand keep a lid on recoil. If a 38 (without the +P)is still too much, then back off to 32H+R.Make sure she understands that it will amost certainly be necessary to keep shooting after the first hit. Revolvers are simple, and she probably won't be able to do a reload anyway.

If both of you are comfortable with semis, then 9mm is the way to go.
40, 357 sig, 45acp, and 45gap are better, but only if she is comfortable with them and can shoot them well....quickly. A slim45 gap might just steal the ladies share of CC hardware in the future. Short case=smaller grip, if the cartridge survives.

The 9s and 40sare so small now that the 380has just about lost its market niche. I wouldn't bother. If I need to hide something smaller than a 9mm, the next step down for me is a 32acp, and the bad guy won't know I have it until he feels the point blank muzzle blast on his skin, and about the time his mistake registers, I'll be finishing off the mag. Little calibers need followup without hesitation.