Changing majors at TAMU is not difficult for students with high grades. Some majors (like Business, which requires you to change to General Academics before switching in) are harder to transfer into, so she might think about starting where she is most confident. You can find good information on changing majors at https://us.tamu.edu/Students/Change-of-Major.
Double majoring is a little trickier. Mays Business School doesn’t allow double majors outside of the college, so if by “CS” you mean computer science, she couldn’t double major in both.
Minoring is easier. You can find more information about that here: http://registrar.tamu.edu/Graduation-Degree-Programs/Adding-Removing-a-Minor
Both Public Health (http://catalog.tamu.edu/undergraduate/public-health/bs/#programrequirementstext) and Environmental Science (we have two close majors, Bioevironmental Science and Environmental Studies - http://catalog.tamu.edu/undergraduate/agriculture-life-sciences/plant-pathology-microbiology/#majorstext) are going to be biology and chemistry heavy. However, if your daughter has interests in this direction, and is interested in business, she might check out the UNIVERSITY STUDIES - BS, ENVIRONMENTAL BUSINESS CONCENTRATION degree. (Also, if she is, please have her contact me about the Udall Scholarship - http://tx.ag/NatlFellows).
Linguistics courses at TAMU are taught in the department of English. If she’s interested in computational linguistics, though, she wants computer science: http://nlp.cs.tamu.edu/index.html, perhaps with a minor in English.
We’re just starting the review process for the University Honors Program. If your daughter completed her application, we’ll have responses back by the end of March.