Most would say a 9mm.
From that I would suggest a Glock 19, 26, or 43. They’re simple to use, simple to take down as well.
Any of the CZ, Smith and Wesson, or maybe Springfield competitors to Glock is good too. Sig also makes excellent handguns but in my experience feel heavier than a Glock or CZ. And are usually more expensive.
.380 Bersa Thunders are nice guns too, they look like the James Bond gun, but .380 is an iffy self defense round for a lot of people.
that’s my 2 cents on the matter. The 26 is smaller than the 19 so it might fit your wife’s hands better