Congrats on saving that beautiful betta! If you can get vitachem, garlic guard and/or fish protector you can add those daily to his tank to boost his system. Look for a product called rid-fungus to have on hand if he gets a fungus in the future. It's organic, herbal and safe to use, even...
I'd report that shop and that heartless person to the authorities.. the place should be closed and she should be thrown in prison.
Unfortunately, I don't know of any way to treat tumors in bettas. We've had a few with those who lived a happy life for over a year.
Welcome to fishlore and best of luck with yor betta when the tank is cycled. I'd definitely not put tetras in with a betta. If you can find java moss or similar plants, or plants with big broad leaves, bettas like those.