Answer:
In such a scenario, it is recommended that Broker A should acknowledge the concerns of the customer because this is a material concern. They should then remind the customer that there exists a Property Disclosure Form which is made by the Seller that the customer is allowed to view to see if there is anything wrong with the house.
You as the broker, are acting on behalf of the seller so should not disclose any information than what is on the Property Disclosure Form.
If however, you know of a radon problem in the property in your personal capacity, the law requires that in the interest of safety, you reveal this to the customer if the seller did not disclose this in the form.