Keep her seperated for a month. If I bring new birds in I like to do at least two at a time so they don't get so lonesome in quarantine and so they have a buddy when it's time to integrate them into the main flock, but even though it's harder with a single don't shortchange the quarantine, it's just not worth the risk to your main flock.
You'll want to keep a close eye on her stools, general behaviour, and it's also a good idea to check her over closely with a flashlight at night looking for mites and lice. Pay particular attention to under the wings and around her fuzzy butt. Mites are easier to see/more active at night.