Note that it is not a given that ND is better than tOSU for math. Some rankings (although at the graduate level) have tOSU higher than ND.
Is her goal continuation to PhD study in math, or more preprofessional at the BA/BS level (actuarial, finance, statistics, operations research, computing, etc.)? The answer may help you determine what to look for if you check course catalogs and faculty rosters to see how well each school’s math department (and related departments like statistics, computer science, economics, industrial engineering) fit her interests.