swollowing too much water isn't an issue with fish, since they breathe it and swollow it regularly. Has your friend tested the water parameters? what are the readings for amonia, nitrite & nitrates? how long has the tank been running? how long has the fish been in it? what is the water change schedule? I cannot do a search on fish diseases from my phone all that easily, but try a search for floating goldfish... does it seem bloated at all? What are some of the other symptoms?
I'd suspect swim bladder disease. Usually triggered by poor water quality, it's often too late the save the fish by the time symptoms are advanced. Test the water, perform appropriate water changes immediately, and try feeding peas.