You’re the Expert
Is there anything more annoying than your friend’s younger brother? Even though he isn’t your brother, the relationship often has a similar dynamic. So when he acts up or is annoying, you don’t mind reacting as if he were your brother.
In situations like these, it becomes clear how annoying they can be. This little punk is trying to tell you how your job works, and that’s not something you can easily tolerate. It would be different if he were just trying to converse with her, but that wasn’t the case in this situation.