I have a question. Did you help put the stands or do any work to assist in this yrs hunt, also did he put any size restrictions on you b4 yesterdays hunt?
The reason I ask is I had a similar situation happen a few yrs back, my best friend and I worked all summer long to put up stands and helped each other out for our "best" hunting spots. We had agreed that we could both hunt ea. others stands except the one stand that was our "sweet spot". Well needless to say, he hunted one of my other stands and kills a nice, wide 8pt. At 1st I was a little ticked off, but we had ea. spoken about this and he reminded me of it. We are still best friends today.
My suggestion is if he's truely your best friend and you want to continue to hunt w/him, it would be to have your hunting rules in place and know where you stand w/ea. other going forward. Tell him its your land and I'll honor your request this yr but if you don't kill him and Im going to continue to hunt w/you and help hang stands and work the property then starting next yr everything is fair game. If he cant agree w/that then Id be out of there looking for another place to hunt. JM .02 cents worth.

BTW, I don't agree w/what he said and is doing. I think Id have to find another place to hunt but w/hunting season upon us its going to be very hard to find a place to hunt.