While i voted that the customer was in the wrong here. I do believe that all pet shops in the UK, including our favourite LFS's have a duty of care to the livestock they are selling. They can and should refuse sale of fish if they believe they are going to be mistreated or not cared for properly. While coming out with "You do know this is a salt water fish don't you" is a bit patronising, they could have asked what the specific gravity of the customers water was. They would have been instantly confused, possibly giving an answer like 50 ppm, betraying the fact they only had a fresh water tank. That said, retail in general is a nightmare from the retailers point of view. Having worked in a Jewellers and seen some of the fast ones that customers try to catch you off guard with, I wouldn't be surprised at anything, including people buying the wrong fish for their tank. That said, how many LFS's blatently lie to get customers to buy their fish, like selling a small pleco to someone with a 20 gallon tank?!
I put off keeping tropicals for ages as i believed they were really hard to keep. Before i got my first fish, even the tank, i did loads of research! Books, talking to various LFS's, TFF, Pracktical Fishkeeping etc... I did consider keeping marines too but the cost of the initial setup put me off. It's not hard to find out about a fish before you buy it, just put regal tank (i think that's what the fish was) in Google and you've got more information than you need to let you know what water the fish prefers.
Ok, after a rambling post, I agree with the majority, the customer was to blame.