Moniq, how long has he been sick and not eating? Have they tested the water for ammonia, nitrites, nitrates and/or pH? What is the tank temperature (if above 24 degrees they have to bring the temperature down below that if he remains in it).
Might pay to put him in the fridge it might help (any foodsafe plastic container with a lid that will fit him). Only place in fridge if the temperature is 5degrees celcius or higher. If unsure, test by putting a glass of water with thermometer in the back of the fridge; check in 30 minutes.
Then Fill container with dechlorinated water, place him in it, cover with lid, wrap container in teatowel (to stop temperature fluctuations and light stressing him as the door opens and closes).
They will need to do complete daily waterchanges, so make sure they either place a bottle of dechlorinated water or another container of dechlorinated water in at the same time as the one he's in so they can move him from one to the other next day, cleaning out the old one and filling for next day change.
Apart from his thinness and lack of appetite what are other symptoms?
Also, are there any other tankmates?
If they don't have the tests for ammonia, nitrite and nitrate - take a sample of water to local petshop and ask them to test it and write down the figures.
One thing, make sure they don't use any fish medicines as advised by petshop, as some fish medicines can be toxic to axies.
