Look at the relative rankings of the math grad programs at each U. She likely will take some grad level classes for her honors degree. USN&WR ranks OSU #28 and ND #41. That is probably a significant difference in caliber. Why pay a lot more for a less prestigious math program?
She definitely needs to consider other factors as well. Her education is much more than just the academics (Ohio State wins on that account, btw). If she really would love the Notre Dame culture you may be able to justify the extra costs. However, Honors programs at state flagships, especially the top tier ones (OSU is in that group) give peer groups on par with many top tier private schools.
For the best MATH education of the two go with OSU. btw, OSU also outranks ND by even more for computer science a field she may also wish to take courses in.
The overall prestige of a school does not mean it is the best for all subjects. In the world of mathematics she may be eligible for more opportunities at Ohio State.