Okay I talked to a friend of mine that knows more about them. I told him exactly what you've said so far and he responded with this:
"Well, first you gotta find out WHY they changed the temps, and WHY they are misting a watersnake. Also, need to know what they have been trying to feed it. Is it a wild caught, or is it captive.
The only reason to lower temps would be in attempts to trigger breeding. The misting doesn't make any sense at all, they like to be dry when they aren't in water, other wise they are prone to moisture blisters. Also, wild caught snakes tend to eat more amphibians when temps are cooler then fish that head deeper and are harder to find during cooler temps, where as, frogs become more available around water sources during cooler times.
Also, depending on what it's been fed, it could be a parasite. They said the vet treated it for bacterial infections, but if they've been feeding is a lot of live food items, it could be parasitic, or if they've fed it solely goldfish, it could be a vitamin B defficiency.
Just too many variables and not enough information."
If you can tell us more information, I can relay it back to him and see if we can get things settled for you.