Some bettas really do have a strong immune system and can live in those for years, as long as they do water changes.
My friend got one of those 1/2gal hex tanks that you can divide in half for her kids. Can you imagine that? 1/2 gal for 2 fish!
I told her that was wayyyy too small, "but its specifically for bettas!" she said. I didn't wanna argue, that doesn't help. Eventually one of them got fin rot, so I offered to take her and heal her, I never brought her back!
, for the other one, I bought the kids a 2.5 gal pirate ship themed one and put her in there. I go to her house every week to do the water change...because my friend never does!.......I would kidnap that one too, but the kids are attached to her now.