The right thing to do in my mind is to have someone contact the guy who found and took the buck. If I understand your post correctly, you would have recovered the animal the next day if the other guy wouldn't have taken it the night before. If the guy refuses to turn the animal over to the shooter who intends on tagging said animal then I would notify the current possessor that you intend to notify the proper authorities about the incident and the untagged animal....if your friend has no intention of taggin this animal then by law he has no right to it either and the current possessor is still in possession of a unlawful untagged animal.

You really have two choices.....forget about it, or in my book, do the right thing. BUT the more time you procrastinate on the matter the harder it is going to be for you to prove that you lawfully took this animal and intended on recovery and tagging of said animal. The evidence will unfortunately go missing.