Should I?

I have a pet goldfish named Lola and she's one of those large headed goldfish. Lola has been sick for a couple of weeks.There 2 other goldfish sharing her tank (they belong to my housemate) and these 2 devils have been biting Lola's tails off!! Of course we didn't notice it at 1st and now, her tail is so short, she can't even float let alone swim!Most of the time now she gonna have her head straight down since it's heavy and she cannot float anymore.I seperated her from those 2 devils but she's becoming more sick .She rarely moves and now i noticed that the side of the body starts to peel off!!Well, i've been thinkin, should i put her out of her misery or just let her be like that? It's really sad to see her like this...What should i do? :( Any fish experts out there?Need help!!