You will need to maintain the strictest biosecurity to keep that bird from potentially infecting your flock.
I would not give her anything until you are sure she has something that needs treatment. I don't treat birds prophylactically because it can make diagnosing them much more difficult when they finally become symptomatic. Give her access to feed and water, and monitor her closely. She will either improve or get much worse in short order. If she gets worse you can decide what to do when you know more about what she has.
If she should improve, you are going to need to decide what you are going to do with her. I would not introduce her directly into your flock. I would introduce a sacrificial bird into her quarantine space and see if that bird gets sick. If the sacrificial bird remains healthy for a month after sharing the same space with the leghorn, then she is probably healthy and can be added to your flock. If the sacrificial bird gets sick, then you know the leghorn is a carrier of some dread disease and should not be introduced into your flock.