Well how old was the snake when you got it? Ribbon snakes normally only live around 10-12 years. If the other snake in the same cage seems fine, then I'd say it's something genetic or age-related but there's no way to tell for sure until you talk to a vet. Like the others, I would also strongly suggest you remove the other snake from the tank if you haven't done so already.
I know it can be hard to find good reptile vets. Try searching for them online. There might be some around that your vet doesn't know about.
That's about all I can suggest. I know it's horrible having to see an animal suffer and I hope everything works out one way or the other. I am sorry though![]()