Do you know why the hen passed away last week? When one has CRD they all have it, whether they show symptoms or not. It usually will flare up in times of stress. Have your chickens been wormed and checked for mites and lice lately? Has this sick hen lost any weight? Since you said she seems a bit lethargic it would probably be a good idea to get some vitamins into her (I like nutri-drench or poultry cell) and make sure she is eating and drinking well.