I was a chief flight instructor at a flight school located on an airport that had these silly rules about "rogue" instructors not affiliated with the ONLY flight school on the airport - the one I ran. I had no problem with the one or two individuals who taught their friends/family/the lone student who didn't want to pay us. We still made a profit, small that it was, and managed just fine. Even got a few students from the "rogue" instructor when he was too busy. Competition is just fine and if all you're going to do is train your friend, you truly are not competition to the "airport sanctioned" flight training facilities.