The aviary dept personnel may be able to give you a clue what it is and offer advice about your other hen if you bring her. If you decide to keep her, make sure you isolate her until you are positive that what she has is not contagious. And practice strict bio-security. Do you have any Oxine? If...
It really is a judgement call, but if she is not in pain or suffering, I would give her a chance. Isolate her from the rest of your flock and treat her according to what the necropsy results reveal. Are you taking the other to the vet for the necropsy? Can you take her along and get an opinion...