It is highly possible/likely that your other birds are going to come down with whatever this is in time, they have already been exposed to this bird. Quarantine at this point is almost pointless but I won't say don't do it.
Watch and listen to her very carefully. When birds have any kind of respiratory disease, doesn't matter if it's bacterial or viral, their odd's of developing pneumonia/secondary bacterial infections is very, very high. That's why many people treat respiratory disease outbreaks immediately with antibiotic's. You can reduce or remove the risk of pneumonia and infections and birds recover faster and with far fewer complications. They can usually recover from these diseases, it's the complications that most often kill them. Though, as mentioned, they will remain carriers.
If you bird starts coughing more or you hear any wheezing or rattling when she breaths you may want to switch to Tylan 50, either injected or orally, rather then dosing in the water. Dosing birds via water is hit and miss at best. You are delivering low doses of medication randomly and sick birds often drink less then normal. Injecting or oral dosing is a much more sure way of getting the proper daily dose into them.