How can her grandma have obtained the guardianship paperwork out of the blue and what do we do to revoke it?

My girlfriend’s grandma has never had custody or guardianship over here. When she moved in with me after almost a year, her grandma somehow in just a few hours got guardianship of her and filed a runaway report to the juvenile court and had her arrested. With both her parents living in another state. Her grandma is a notary of public.