Just something to think about, Betta's can survive a month or so without food so I doubt it was the not feeding him that killed him.
I never leave any of my friends or neighbors in care of my fish just because I know something like this would happen and my luck they would end up with some horrible disease so only family get's to take care of my fish. But all in the same, I'm sorry he died S.I.P. little guy, I hope he's happy swimming under the rainbow bridge :)
So for disinfecting, here are some steps.
- Find some Clorox Bleach with no other additives.
- Make a solution of 1 parts Bleach and 19 parts water in your tank after you take the decor out. You can let it sit for about an hour or so, which ever you feel comfortable with. (For extreme cases I let this process span over a few days but this was just death by fin rot most likely, but never bad to be on the safe side)
- After that, pour of the solution, rinse out with tap water until you no longer smell the bleach.
- After that I usually fill up with water again and pour in 4x the amount of conditioner and let that sit for a few hours to become tank friendly again.
- I do the same thing with all my equipment, decor and other stuff that have come in contact with him.
You can also use white vinegar, swish that around your tank, I take a paper towel and soak up the excess and wipe down the walls of the tank. Again keep rinsing until you no longer smell vinegar.