Copied straight from my barn rules:
11. No visitors allowed in stalls. Boarders may retrieve their horses from the stalls and properly tie their horses for visitors to pet. No outside riders permitted.
12. No visitors allowed in pasture. Boarders may enter the pasture to retrieve their own horse, ONLY.
13. Boarders may not handle any other equine than their own.
15. Do not give treats to any equine other than your own unless you have written permission to do so on file with the stable.
My boarders all understand that I would not allow a friend of theirs to show up and handle their horse. I won't accept the liability. In the case of an out of state boarder, I'd welcome a FAMILY member to check on their horse but not just a friend who decided to show up.