FW Fire Eel, Death

I was also under the impression it was a saltwater tank as well ,with the snowflake eel, orange spot ray, catfish and crabs, and assumed the shrimp and goldfish were food.

 

shrimp and goldfish are food, but everything in the tank is FW breeds.

 

Gold fish are one of the worst things to feed any kind of fish whether it be salt or fresh as they have very little nutritional value. First wrong move! Then, the addition of living things way to prematurely without properly curing the water(cycling), was probably the deciding factor in everything dying, then further disrupting the water chemistry even more as things continued to die.
Research, do things slow and in moderation is the best advice I can give your friend so that this doesn't happen in the future!