The saying "Character is what you do when no one is watching" comes to mind here, UA needs to change they way they do this. Some points for actually responding to customer feedback but there simply has to be someway to observe the service without making it obvious. I suppose its great for the folks on that 1 flight, the crew knows they are being watched and act accordingly, if anyone thinks that the nanosecond the onboard sup walks out the door things won't return to normal they are a fool. And since it was the "normal" that brought the supervisor there in the first place this in no way solves the problem long term.
Its can't be that difficult to have someone appear on the manifest as revenue and not tell they are an onboard supervisor and take notes without making yourself known to the crew so they can see how they interact with customers when they don't know they're being watched.