I would go to the farm store, and pick up some Tylan, which will cost around $30 to $35, but it really will knock out any respiratory illness she has. I like the powder form. You make a gallon at a time, and you just put that in her drinking water. If you're worried that she's not drinking, give her some with a dropper.
Pricey, but great stuff. Now, if you can find Denegard, that's another great product, it's in a big jug, and advertised for swine, but it's great, plus chickens don't become resistant to it, because it's not an antibotic like Tylan is.
Good luck with your hen,
Sharon