Only do a regular water change... 100% is not necessary. Sometimes betas just don't live very long. Often times, the reason is the way they are shipped (it is horrible). If your other fish is doing fine, and your tank has been running for some time, I would have to say that it is simply old age, or just a premature death from shipping stress.
Someone else might pipe in on this, but you could probably go out and buy another one if your still interested, or take down your divider, and go get 5 or 6 neons, to liven the tank up!