There are numerous reasons the Feds could ask for a 709, but in your case there is a need to ensure you are providing the quality of instruction the public is paying for, that is to prepare the student for his check ride. You need to look at this as the Feds doing their job, not out to get you. And it is true, a compliant attitude, along with understanding all the roles in your line of work is necessary. A lawyer probably will have no effect on the outcome as you will still have to deal with the Feds, but I can guarantee you, taking a stand of no communication will. As a professional, do you believe your instruction MAY need improvement, or you have a role in the outcome of the check ride?