Pawn Shops
Shotguns and guitars a plenty at Pawn Shops.
Guess you might not know what you are getting into - but buy a named brand like a Mossberg and if you have any issues with it...send it in to them to fix. Especially if it is a more well known (or quantity sold) brand be easier to get parts for - etc.
Also check armslist - I've purchased quite a few firearms from gun stores in different states off Armslist. (I don't mess with private sellers on there)
Some of the older shotguns are super nice....sometimes the price goes down as the barrel length increases on some models..because people want that HD shotgun but not a 28 inch + barrel, so some of them sit on the Pawn Shops walls for awhile...if you can get a good deal on a shotgun with a long barrel...replace barrel with HD barrel if cost effective or cut it if possible.
When I went to the local pawn shop in the city I used to live in, there were usually a bunch of hunting and pump shotguns on the wall. Of course, this was before the pandemic panic buying surge.
Don't get her a shotgun, get her a pistol caliber carbine, like a CZ EVO Skorpion w/ lights, laser etc. Much easier to shoot and hit things with than a shotgun and many more rounds to fire.
That's not a bad idea either. Though the straight blowback pistol caliber carbines will have a bit of recoil due to the big bolt slamming back and forth during firing.