Hi Clearmark. I'm so sorry about your fish, that's an awful thing to happen. I am curious what your wife used to clean the tanks and the decorations? If she used any sort of soap, there is your problem. No amount of rinsing can fully eliminate the harmful residues soap leaves behind, and it is toxic to fish.
Another thing that might have happened is shock from the change in the water. Usually, when changing water, fish need to be acclimated to the new water. It's best to mix a bit of their current water with the new water, adding more little by little for a while. When I am acclimating bettas to new water, I usually give them no less than an hour to get used to the new water. Too much change too fast can shock the poor little guys.
I do hope you get another betta, they are great little buddies! Feel free to ask as many questions as you can think of on the forum, all of the members are very nice and ready to help! :)
Back after a hiatus.
Pongo - HMPK Koi Male
Onyx - DSH Cat