This is what Posner says in an update interview with Frontline:
What did your investigation show about a possible link between David Ferrie and Oswald?
I used to believe it was unlikely that Oswald and Ferrie ever had any contact at all. But when FRONTLINE discovered a photo of the two in the same group at a Civil Air Patrol gathering in 1955, the evidence shows the two were briefly acquainted with each other. The key question then becomes whether they revived any relationship when, in the summer of 1963, Oswald returned to New Orleans. This is the critical period, because it is only months before JFK's assassination.
But there is no credible evidence that the two encountered each other again during that 1963 summer.