Costa is a great instructor
the S&W M&P Shield E-Z slide, as well as their .9mm, two other popular handguns you could consider.
@cajunchefray: good choice on the S&W model 60 — simple to operate and effectiveYes, let the lady decide which gun feels right to her. This is important. My wife can shoot my 1911 .45 ACP well but is comfortable with a 5 shot .38 Special revolver.
A Smith &Wesson Model 60, or for lighter weight, a 642 DAO, with a smooth trigger pull is essential. For some women, racking the slide of a semi auto can be difficult in a stressful situation.
For a semi-auto choice, the Smith & Wesson MP in .380 is a very good choice, very easy to rack the slide, change mags, and shoot accurately. The Mossberg semiauto equivalent is also an easy to handle pistol.
Personally, I don't prefer these, but the ergonomics are excellent.
The lady must feel confident to handle and shoot the pistol, whether it is a .22 Magnum pocket revolver, or a SIG P365. The Glock 43 is a good choice also.
Crimson Trace Laser sight is a great upgrade. My wife wanted this for her.38 Special and we did. Our first time trying a laser sight , and damn, it really is an advantage within 5 yards.I would also recommend a S&W 642 air weight for self defense. It is a hammerless 5 shot revolver. My wife and sister each have one. In a situation where it would actually be necessary to defend oneself, simpler is better. No safety just pull the trigger. A hammer can sometimes catch and delay the gun being brought to ready and a cocked hammer needs to be made safe if not fired. A crimson trace grip would be a good addition to this for both accuracy and safety.
Hornady makes a critical defense lite round, it has a fast expanding bullet, reduced powder charge that should not leave the body of a perpetrator. A missed shot in a house upon hitting drywall should be less then lethal on the other side.
Lots of good advise on this thread and some difference of option based on personal preferences. Good luck with your decision.