You lack the authority to ask a non-member of your club to comply with any rules. As has been stated, the park officials are the governing entity and unless they were to appoint you as their representative, your wasting your and the other guys time. As the guy is flying 2.4 Ghz equipment, I would just back away. If he was operating on 72Mhz, I would hope he was understanding enough to state the frequency he was on. If he won't comply, then he will likely loose an airplane or two before the importance of a frequency control system is driven home.