Teaching someone who is just not getting it is. Very. Frustrating. A 9yo, especially one with emotional intensity, is likely to cause as much harm as help because of their emotional reactions. It takes maturity.
Also, it's 4/5th grade math. It's a black/white, right/wrong subject. That naturally lends itself to a different mindset that is less tolerant of mistakes. My DW recently pointed out what a sharp difference she notices when I work with my DD9 on math (DD's favorite) versus any other subject. Math is the only one where DD has occasional meltdowns.
So I'd say you can tell the other parent that you don't think it's a good idea because your DD's maturity level may not be at the level necessary to maintain her patience and understanding, and exposure to that might actually harm the other child.
