Shades of "Good Will Hunting", here...
My first answer is to echo what a lot of other posters have said, and help her figure out what it is she wants. If she can find something that she can be truly impassioned about, then she'll have a leg up on most other kids starting uni. Of course, she's still pretty young, so she might not know yet what turns her crank.
On the other hand, I've often said that if I could go back to uni and do another undergrad, I'd do math for sure. There are just so many doors that math leaves open, and the kind of thinking the various maths foster is useful.
On the third hand, if you really want a concrete suggestion other than math/science, I'd go with Linguistics or Cognitive Science (but I may be biased ;). Have her check out the NACS program at Maryland, or CogSci at Johns Hopkins, or Linguistics or CogSci at MIT. Or the Symbolic Systems BA at Stanford (possibly the coolest undergrad program in existence). There's a lot of non-mathy (well, some math is involved), non (hard-)sciencey stuff going on that's really interesting out there.
Best of luck to her. She's at an exciting time in her Life. Help her realize that.