Wardens are pricks, never convince me otherwise, but it has been my experience that there is always something else they can ticket you for, and they really like to threaten you with that, as if they are doing you some huge favor. No doubt your friend was in the wrong with the borrowed tag, but as far as you being "party to poaching" as someone put it, give me a break. I would say pay the fine and don't dispute it though, unless you want to see a lot more of the warden.