I don't think it's just the sheath. From the picture it looks like that but from his description it sounds like the barb itself is detaching without having grown a new one. I had this happen when a female pearl got really beat up by a male. For a while there was no barb at all and the tail didn't look good at all. Then a kind of funky barb grew that didn't look so hot either. After that she started growing normal barbs again.