IMO you should pay the fine. It sounds like you knew he had a tag w/someone else' name on it. Therefor you were part of the crime. If you simply asked him if he had a tag and didn't know that someone else's name on it then things would be different. That would have given you a certain level of deniability.
You can't deny anything at this point. You have already put yourself in a bad situation. I don't inspect anybody's license...... but if put on the spot by the game warden I wouldn't lie about it.
Sure your buddy started it but when confronted by the law you should have come clean. Where I live the CO's have the legal right to confiscate everything that was used in the commission of a crime...... all weapons, other equipment and your veh would be gone. And there would be a good possibility that you would lose your hunting rights for 2yrs. At this point I would count yourself lucky and seriously rethink your friendship. Your name has been tarnished.......
I'm glad you came clean on the web but it ain't going to help. Pay the fine and next time don't put yourself in that situation.
You should have put him on the camera!