I felt awful when a baby guppy swam through my divider (sneaky little fish!), and rushed to get it out, so I really cannot understand why they would knowingly want to fight the fish... The parents really should not be encouraging them, whether it was their own money or not, fish tanks can be returned so it is not an excuse...
I really hope you can talk them round, it's not fair either on the people who want to save a betta from the store but can't as they are being used for fighting by 2 people not fit to look after themselves, let alone a defenceless animal. Makes me sick.
Sorry

, but the very best of luck play