it appears to sound like popeye, but it was probably with another infection, because popeye isn't fatal i had a fish with popeye (died with another reason) i do not know what cause him/her to die probably a fungus with popeye? did your friend clean his/her tank weekly?
pop eye could be easily treated if caught right away, unlike my little fella was neglected and had the disease before i got him. you could use salt (i used table salt :P) to cure it, one of my fish had pop eye now she is back into life again
also give the fish a every 3 day cleaning in the tank, and salt daily.