Dede, considering regular household foam is also not flame retardant, I am asking myself why that would spoil the sale.
Is flame retardant mandatory where you are ? I never heard of residential requirements, but I am in the soggy very green state of Virginia.
Commercial work demands it, certainly. You can get California rated flame resistant regular foam.
Mildew, if it's a problem for her, certainly does not originate in her foam. Think of mildew like fleas. They multiply and spread, but the responsibility is to find the point of origin and remove the breeding entities. Mildew is the same, it is breeding some place other than the foam. It isn't organic. The foam can be a resting place but not a source of food.
It's nice you want her to have the best, and have it all. But, if you can't have all you wanted, indoor foam is what she'd get from someone else. Unless it's required, and then you get the rated indoor stuff, and you still don't need to lose a sale.