There’s definitely no need for a full-blown double-major in biology. If the med school prereqs get her close to a bio minor and she wants to finish the minor, all good, but there’s no real tangible payoff for getting the minor vs. just taking the required courses for med school. I can’t imagine any public health employer prioritizing her resume because of a bio minor, unless it were some unusually bio-heavy job in environmental health or something like that.
For career opportunities in the public health field, as others have said, a strong quant background is a real asset. Even a minor in stats, data science, or data analytics would catch the eye of an employer who’s looking for this skill set. Since public health operates on a population level, data analysis is where the rubber meets the road. Another quant-adjacent minor that a few schools offer (Ohio State is one, and that’s another school that could be worth considering) would be epidemiology.
Overall, if she decides to go the public health route, it could be more advantageous to invest elective time in getting a head start on her MPH, where such pathways are possible, vs. collecting undergrad credentials. Many schools have BA/MPH pathways, which often add only one extra year and aren’t necessarily a bad idea even if med school is still a goal.