It's usually recommended to treat them all since they have all been exposed (if that's what it is). If you are not sure, and only want to treat that one, then you will need to separate her so that she only has medicated water. If she recovers, and another starts looking off, you'll have to repeat with that one.
If it's only the eye bothering her, then that may be all you need to do. I would keep a close eye on her though. If it seems like more than the eye is going on, I would treat.
If it's only the eye bothering her, then that may be all you need to do. I would keep a close eye on her though. If it seems like more than the eye is going on, I would treat.