What does this mean? This sentence is quite unnatural, in my opinion.
It means, has she graduated?
I meant more the given translation, 'Has she already finished her school?' doesn't really make sense, 'Has she already finished school?' would make more sense. Also, isn't 'lulus' used more for graduate?
Yes. Lulus = graduate. Menyelesaikan = finish. Both terms are commonly used in Indonesian, in the context of completing your education.