A passenger wants an exception to a rule and says another airline allowed it. How would you respond?

If a passenger wanted an exception and said another airline allowed it, I would acknowledge the comment without turning it into a comparison argument. I would say, "I understand that may have been your experience, but on this flight I need to follow this policy," and then I would explain the approved options as clearly as I could.

I would keep the explanation short and confident. The more I sound defensive, the more the conversation turns into a debate instead of a decision. I also would not criticize another airline or imply the passenger is lying. That usually only makes people dig in.