I would probably tell him no and explain that you and your some hunt the 15 acrea. With 3 people hunting that amount, it can become over hunted fast.

I would also call the landowner and tell him you told the other guy no and the reason behind it. That way you won't have to feel bad (if it was me, I wouldn't feel bad for the hunter, I just wouldn't want the land owner to think differently of me). If you get bad vibes from the homeowner, then maybe reconsider. But my first priority would be the landowner vs. the hunter.