I can’t imagine a college where an intro econ class is going to stump a math major who is doing well in math.
Finance is a pretty algorithmic discipline- you learn the rules of the road, you apply them and punch the numbers through your formulae, you get the right answer. Econ is much more conceptual-- but again, it’s a logic driven discipline and the actual math in an intro course is going to be pretty much Mrs. Riley’s 6th grade arithmetic.
She should take the class and not worry about the grade. Grad school in a STEM discipline is not going to care about a B in econ. And I suspect she will do much better than a B.
And she will spend the rest of her life voting, paying taxes, buying goods made overseas, etc. very happy that she understands the role of the Fed, why exchange rates matter, what is inflation, etc. due to that econ course.
Every time she looks at a pair of shoes made in Italy which cost $500 sitting near a pair made in Viet Nam which look identical but cost $80 and understands why she will be happy to have taken econ. Every time a candidate promises free college (or whatever the new entitlement is) without raising taxes she will be happy to have taken econ.
Good luck- she sounds terrific.