They probably already have,( since she was in with them for a whole day, and sneezing ) as there is an incubation period for all diseases, it could be a bit of time for others in your flock, to show symptoms. I would treat the whole lot, probably once they start showing symptoms, although someone that has more knowledge than me, would be a better judge of that, and if you are going to reintroduce her, might as well, as your other chickens have already been exposed. The best antibiotic for broad spectrum use is duramycin 10, and tylan, water soluable or injectable ( the injectable can cause necrotic spots at the site of injection so keep an eye on it) are what most people use. Tylan, I read is the best CRD med out there and the duramycin 10 is a tetracycline drug for treating broad spectrum issues when you don't know for sure which respiratory infection they have.
I would try the Dyramycin 10, if that doesn't work, go to the Tylan. The injectable gets in their systems faster, but the water soluble is easier if you have more than a few birds. I don't know the dosages for the meds though, sorry...maybe some of the other more knowledgeable OP's on here will pipe in?
Just so you know, if you keep this flock, any birds you bring in, will have to be isolated( 2-4 weeks is recommended, longer is better ) and vaccinated against whatever your flock has and no birds can leave your property.( called a closed flock ) You should start practicing some biosecurity also, so you are not infecting other flocks...so sorry this happened...but what's done is done, now to treat and get that bird and probably others in your flock, eventually, healthy, than make sure you aren't spreading it to others also...best of luck!!!