Lying during the course of an otherwise lawful detention in order to preserve the peace is understandable. But, that does not seem to be the case here. I would not want to try and defend it, and I KNOW that some officers make dumb decisions out of ego rather than common sense. Such is likely the case here. The officer told them to stay because, darn it, he was in charge and wanted them to stay until his sergeant got there. Oops. Another case of the ego getting in the way of the law.
Yes, there might be a lawful justification for the detention, but it is not obvious from the story as told. And even trying to infer more into it, I find the justification would be weak at best no matter what the officer might claim. If this was indeed a civil issue then about all that is left to be said is: "Sorry, not much I can do, take them to small claims court. Have a good day."
A Nor Cal Cop Sergeant
"Make mine a double mocha ...
And a croissant!"
Walk humbly with your God
-- Courageous, by Casting Crowns