My roo, Perkins, had this a few weeks ago when he was about 10 weeks old. He is all better now....
Medicine - Get some terramycin powder from
Tractor Supply and add it to all their water. I would still seperate for a couple of weeks if possible, but treat the whole flock since they have been exposed. I used 2 teaspoons to a gallon of water for 3 days then reduced it to 1 teaspoon for 10 days. Some people have said they use 1 Tablespoon to a gallon for 2 weeks, but it didn't take that much for mine. The water will have to be changed everyday.
Comfort - If she is having a lot of trouble breathing, you may want to bring her inside for some cool air. I brought mine in for the first two days.
Feed - Give them a little plain unflavored yogurt everyday to replace their tummy bacteria that the antibiotics will destroy. You can sprinkle some crumbles on it to get them started on the yogurt if they have never tried it.
I had heard that holding them upside down for a few seconds could help clear their sinuses, so I tried it. The first time I did it something came out of his nose (and onto my shirt) that looked like soft butter. (gross!).

I bet that felt better out than in though. Anyway, nothing came out when I did it a few days later.
I think my roo had a mild case...if the breathing is rattling or there is a very bad strong smell coming from the mouth, then you may be dealing with CRD or Coryza.
If you do these things and it doesn't seem to be working after three days, then keep asking for help. There is no guarantee this will work for your hen.
Good luck and I hope she feels better soon.:aww
~Rebecca