Make it a learning lesson that's for sure. Some great friendships have been lost over much less I am sure. The one thing that bothers me is there is no way that deer could have been mistaken for anything even remotely close to what you are managing for. So either you guys did a terrible job teaching him or he said to himself your rules don't matter. While looking at the flip side of the coin, many have said they would not ruin a friendship over this.....well, your buddy could have though the same thing before he shot. Not sure why it is only up to you to be the true friend and turn your cheek, he owns something in this as well. Either way I am sure the deer will be used and enjoyed and at this point that is most important.
Hopefully we all learn a lesson here as well.