How can you help a dying Betta splendens fish?
I don’t believe in euthanasia, for animals just as for humans. I do believe in dying with dignity which means not surrounded by other fish pecking his eyes out before he is even dead. So I would give the fish normal conditions in which it would be comfortable if healthy, low light, and solitude. Once it is really dead you can feed the body to other fish, flush it down the toilet, whatever. But the life, while it is there, is sacred, and should have dignity, even for a fish.
Whatever science could say about its neurones and synapses, he was your pet.