When I either move a new one in or reintegrate someone back in to a flock, I always sprinkle everyone with garlic powder, and sprinkle the coop with garlic powder, generously both on the birds and the coop, so that it basically knocks you over with theggarlic smell, lol, and sprinkle the garlic...
My reasoning for removing her from the flock is to be able to better control what she eats and drinks, rather than having to grab her and forcefeed a specific dose of whichever immunity boost mix you need her to take that day. It also allows you to see more specifically if she has anything close...
Well, first, I would have pulled her out and kept her separated from the rest of the flock, in case whatever it was is contagious. It would still be a good dea to do so, now, so you can keep a closer eye on her and potentially treat her, if needed.
Have you checked for any lice or mites...