I feel your pain. Had the same thing happen in our camp. An invited member decided to invite his boss. Had lots of hard feelings and I lost a friend. But he was out of line.
However you choose to do this, your friendship with your pal is going to be damaged. You will need to step up and be the bad guy, even though you are in the right.
You need to take the high ground and get control of this situation. Before you have a heart to heart with your buddy, you need to inform the landowner about what is going on. Take the lead on this and get his support because technically you cannot kick someone off the land without something that says you are acting as an agent for the owner. At least that is the way it works here in my state.
Once you having backing of the landowner in writing then you can go about fixing the problem. From what I've read, YOU got permission from the landowner for one other person to hunt the land, not two. Tell your buddy that the other guys has to go or you will have to kick both of them off.
I would do this if your relationship with your landowner was good. Sit down with him, be totally honest. Tell him that you screwed up and you want to fix it back to the way it was in the beginning. Nobody hunting but you.
If your buddy was a true friend, he would never have done this to you.