The damsel's problems are likely related to having been used to cycle an aquarium. The stress can bring on any number of diseases that the immune system normally fights off.
I really wish I could help you, but it seems your LFS
doesn't have any medications, by the time you mail order them it might be too late.
All you can really do is prepare lots of extra saltwater (let it sit 24 hours with an air stone or air pump after mixing) and do lots of extra water changes. Clean water can be helpful in these situations, reducing stress and removing excess pathogens. Feed him the best foods you have available, and hope his immune system will fight this off.