The best advice has been given -- take her to try out as many guns as she possibly can. Shooting different guns can be fun, but tiring too. Tip:
Take a notebook or 5x7 notepad to the range when test firing guns. Have her write down her impressions of each gun. This way, she can recall which ones she DID like versus the ones she didn't. Or which one had the hard-to-reach safety. After shooting five or six guns, two days later you can't remember which one recoiled too much.
Things to avoid in general:
- 2" snubbie revolvers: Hard to aim & shoot accurately
- Very lightweight guns (under 23 oz): Increases felt recoil
- Weak cartridges: .22, .25, .32ACP, .32 Long
Things to help her with:
- Proper fit to her hand
- Use of the magazine & slide release controls
- How to manipulate the safety/decocker
- Slide operation (new guns will be stiffer to rack the slide)
For your $400 price range, you should be able to find a very nice "previously owned" gun that is still reliable and accurate.
For what it's worth, here are a few examples of my handguns that women have found comfortable to shoot;
Top: Beretta 92FS, 9mm; Bottom S&W Model 39-2, 9mm
The Beretta's wide grip made it easy for taller women to obtain a firm grip and it's evenly balanced in the hand. This same idea applies to the Taurus PT92 series.
The S&W 39-2 (and it's follow on pistols: 439, 539, 639) are full size single-stacks. Women liked the lighter weight, the good grip they could maintain. Some said recoil was "brisk" but quick to recover from.
For a .38 Special revolver, there is always the simple solution of a Model 10 Military & Police revolver with 2" or 4" barrel. There are a variety of grips in different sizes and shapes to help suit her hand that are easily available.
If you run across a S&W Model 15, in either form shown here, these are excellent defense revolvers. The 2" will be harder to aim & shoot accurately, but the adjustable sights are easier to use. (This 2" snubbie will shoot 2.5" at 25 yards, just not with my old eyes.)
Another option is the venerable Model 19 (or 66 in stainless) with the 2.5" barrel. More sighting plane, easier to aim & shoot and it give you the option of using .357 Magnums if she wants.
It's fairly heavy, but that helps dampen the recoil. With .38 Special +P, it's fun to shoot. Speer's 135gr .357 Magnum is a mild load perfect for the short Model 19.
Comparable guns from Ruger would be their older Security-Six and Speed-Six lines or the GP-100 series.
Wild ideas
Do not rule out the .45 ACP cartridge. Let her try a 1911 or other type of .45. She might surprise you! If possible, DO try either a S&W 625 or a 3-inch Model 25 revolver in .45ACP. These are heavy, large frame guns but they dampen the recoil to about that of a standard .38 special. These might be out of your price range, but a used specimen may only be a little more.
Beretta made a couple of fairly large .380 pistols. The Model 84 carried 13 rounds double-stacked. The Model 85 is a single-stack and the Model 86 (shown below) included a tip-up barrel for easier loading of the first round (no slide operation required). New prices on these were around $600+ but used guns can often be found for much less.