You might consider looking into the Smith & Wesson third-generation pistols, in the 3900, 5900 and 6900 series.I personally have no issue with her using an older gun as long as it's something she's comfortable with and has the ability to actually come up with replacement parts.
The 3G series is discontinued, but there were many of them on the market and lots of police/sheriff departments across the country used them as duty weapons. They're known to be incredibly reliable, durable. Because so many were built, generally parts and finding a competent gunsmith to work on them isn't a problem. Many can be found for under your $400 limit.
Check the 3913 variants, including the 3953 and 3954 DAO models. Great pistols. And they can be excellent for smaller hands, too. Particularly the 3953/3954 DAO models, which have no manual thumb safety on the slide to fumble with or reach.
For the 3953, for example, you can find several police trade-ins available in GunsAmerica listings. Spare magazines are readily available at the online "parts" sites (ie, MidwayUSA, Brownell's), and replacement springs are available through Wolff Gunsprings.