I board horses & give lessons but if someone wants to bring a friend or family member out to ride their horse I don't care. Sign a waiver & have fun. It's their horse to do with as they please. I have very good, responsible boarders & luckily I don't need to keep a hawk's eye on them.
A boarding barn should be a place to have fun & relax, not worry about rules. Common sense is usually all that is needed.
I'd move too.