I totally know what you are going through! When I was in high school I use to breed rabbits for show, and a friend of mine was a huge copy cat and decided she wanted to breed rabbits for show, so she got some rabbits, didn't know a thing about how to properly care for said rabbits, and it was sad!
I would go over to her house and they would have old stale food and no water or hay!!! I'd get mad and say, why don't they have water and it was always the same story "oh they must have just ran out" So I'd give them water and they'd act like they hadn't had any for days!
Sadly a dog got into her yard and killed them all, but luckily she decided not to get more.
i'd offer to buy the betta from her and see if she is willing to give it up, if not turn her in for animal cruelty. That's what I should have done for my friends poor bunnies.