Hmm, I don't know what to say!

I think that if they have kept the snake alive for 10 years they may be a little skeptical about changing anything they have been doing (I can think of plenty of people who have had kept snakes/lizards in improper conditions and when you try to tell them correct information they just kind of brush it off like they are experts on the situation). I would just suggest printing out some updated care sheets and letting them know that the changes they would have to make are really not that big of a deal! Maybe even suggest a quick visit to a vet if the fast continues (they vet might be able to change their mind about husbandry issues too, if they go to an experienced exotics vet). Even sending them a link to this site may change their mind, once they hear all the information experienced keepers/breeders can give them.

As for the water dish, the reason its getting knocked over so much is because the snake is big enough to push it around, let them know that if they get a larger (ceramic) dish they probably won't have to worry about cleaning up water that gets spilled.

Good luck!