The question is if she will apply to colleges, get in, then defer a year; or if she will finish high school, do the gap year, then apply to colleges. It matters alot the order.
If she is going to finish high school and do a gap year, then apply, you could build into it retaking the SAT or a test like that. Not alot of kids do a gap year before college; you see that more after college when applying for graduate school.
If you just graduate from high school and apply, her application may be stronger if you wait a year after doing something interesting and/or retaking any tests that might need to be improved.
