I don't know of anything at the $100 range worth recommending. Cheapest gun I know of that people say is surprisingly reliable is a Hi-Point, at a bit under $200. Take your pick as to caliber and size. I've never handled or shot one, but like I said, people say they're actually pretty decent.
Next price point I'd point a friend to would be the Ruger LCP at around $225 depending where you get it. Again, never shot one, but handled one and they're comfortable. I'm told shooting one, however, is
not comfortable. Makes sense considering this is a carry gun and not a range toy. But the quality is good and Ruger will take care of her if she ever has a problem with it, so it's worth considering.
Next is the Ruger SR9E at around $350. Shot these a couple times and while not my favorite gun in the world, it was totally functional and the price is right. Solid choice IMO.
If she would be willing to jump in quickly she can get a used S&W M&P9 from Bud's for $350-ish. I think that would be the no-brainer option, even over a new Ruger SR9E. Bud's frequently has used M&P40s for sale, but much more rarely has the 9mm counterpart. If she prefers 9mm, I'd jump on these. They come with 3x 17-round mags, guns are 70% condition or better. Can't beat it.
https://www.budsgunshop.com/catalog...e+Trade-Ins/Used+S&W+M&P9+9MM+W3+17RD+MAGS+NS
That's about it for me without going over $400. I know there are some fine revolvers to be had under that price, including used offerings, from both S&W and Ruger. But I don't know enough about revolvers and their prices to recommend anything. I'm sure someone else can chime in.
Quick note, it'd be good for her to test fire all or some of these at a rental range before buying one, so she can ensure she gets something that suits her well.