Based on the timing of the OP’s question, it seems likely the applicant hasn’t yet been accepted to Michigan, which hasn’t yet released its EA decisions. I’d say wait to see whether he’s admitted, and if so what his FA offer looks like.
For an in-state student who is intent on staying in-state after graduating and pursuing a career as an accountant or actuary, a degree from CMU is probably just fine. The school is well-known and reasonably well-respected in the state of Michigan, and it has its own alumni network concentrated there. But overall the University of Michigan’s math department is much stronger–one of the best in the country, in fact—and as a previous poster noted, mathematically talented students often find their interests gravitate toward other areas of math. And talented math majors from schools with as strong a reputation as Michigan’s often find they have a wealth of professional opportunities beyond accounting or actuarial work, though those career paths also remain open to them. Michigan’s stronger national and international reputation and alumni network would also likely have a bigger payoff for a student looking for a career outside the state of Michigan, especially one “aiming high” for the most competitive positions.
Short-term costs are one thing. Long-term career prospects are another. In general, Michigan grads earn more than CMU grads. The U.S. Department of Education’s “College Scorecard” reports that 10 years after entering college (so roughly at age 28), the average Michigan grad makes $63,400, while the average CMU grad makes $41,900. At that rate it would take about 4 years of earnings to make up for the difference in cost between full in-state COA at Michigan and net COA after a full-tuition scholarship at CMU. So net lifetime earnings would clearly be higher for the Michigan grad. But those figures are averages across all fields of study; it could be that for an accountant or actuary, the gap wouldn’t be that wide.