The man had apparently posted the picture of a schoolgirl in a short skirt with a sexually suggestive caption, and then removed the intern from the chat group when she objected.
His action was captured in time and went viral on the net.
On Tuesday, executives of the chain restaurant were told of the situation and launched an immediate investigation.
Upon learning the story was true, the company apologised and immediately sacked the manager. They have also guaranteed that the victim will be able to complete her internship uninterrupted.