The snake is probably fine (no promises I guess). Mites in themselves don't make a snake sick. I don't think you should give up quite yet! My first snake, also a kingsnake, had mites. She got over them with the method listed below and is today healthy and very cute. I think that almost all petstore snakes have mites, even though there's no excuse for it.
You have to give the snake some sort of bath and completely clean and scrub out the cage. The snake could probably be bathed in a weak soap solution, but make sure to wash it off. There are a few over the counter mite bath products that work too, that's what I used in this case. Some people have reported bad side effects with mineral oil, especially if not washed off. Then you can toss the cage bedding and wash the cage and fixtures in a 1/10 bleach solution. Then rinse that out so it doesn't smell like bleach anymore. This should take care of all the mites, but you still have to watch for a while. I'd keep the snake on newpaper until you're sure they're gone. You'll often find them in the water dish, so that's a good place to look.
I'm sure there's a shot for mites, but you'd still have to clean the cage out completely. The vet can definately give you an ivermectin spray, which is supposed to work very well, and if you can get that, I'd do it. . There are also some products like Provent-a-mite and Black Knight that don't require baths and cleaning the cage. Be careful if you own more than one reptile, mites spread very easily.
Good Luck!
