Oh dear. I'm sorry. It would be tough to have to live with that right in your house! But, sometimes, no matter how much you try to convince someone, it just doesn't work. Especially if they're stubborn and think that they're always right!
And the stories of what happens in stores sometimes are just terrible! Just today I was at the pet store and saw a Betta in with the goldfish! I mentioned it to a worker, but she just shrugged me off and said that they were okay living together. Then I saw them most gorgeous CT, but my tank isn't cycled yet, so I had to leave him.