Requirements: http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Astronaut_Requirements.html
Wanting to be an astronaut isn’t enough criteria to choose a major. Is she the sort of space junkie who follows everything NASA, ESA, SpaceX, etc do and would love to work there? If so, aeronautical or mechanical engineering are good choices. However, it’s very difficult to get into one of these places as an engineer-- even harder to be selected as an astronaut. I agree with the Air Force ROTC suggestion to get pilot hours and cheaper tuition. She should also consider if she would be okay with working in regular aeronautics if she can’t find a job in the space industry.