It's difficult to recognize anyone in those grainy b&w films and photos. It's not Ruby, though, because he was in the presence of several witnesses several blocks away.
WC:
Jack Ruby learned of the shooting of President Kennedy while in the second-floor advertising offices of the Dallas Morning News, five blocks from the Texas School Book Depository, where he had come Friday morning to place regular weekend advertisements for his two nightclubs.849 On arriving at the newspaper building at about 11 or 11:30 a.m., he talked briefly with two newspaper employees concerning some diet pills he had recommended to them.850 Ruby then went to the office of Morning News columnist, Tony Zoppi, where he states he obtained a brochure on his new master of ceremonies that he wanted to use in preparing copy for his advertisements. 851 Proceeding to the advertising department, he spoke with advertising employee Don Campbell from about noon until 12:25 p.m. when Campbell left the office.852 In addition to the business at hand, much of the conversation concerned Ruby's unhappiness over the financial condition of his clubs and his professed ability to handle the physical fights which arose in connection with the clubs.583 According to Campbell, Ruby did not mention the Presidential motorcade nor did he display any unusual behavior.854
About 10 minutes after the President had been shot but before word had spread to the second floor, John Newnam, an advertising department employee, observed Ruby sitting at the same spot where Campbell had left him.