Barristan was on the Small Council in Season 1 of the show. Remember how only he and Ned argued against sending an assassin to kill Dany? It's apparent that Barristan is lying to Jorah because he doesn't want Jorah to know that he knows that Jorah was a spy for Varys. Edit: Apologies for my error. I re-watched the small council scene (Season 1, episode 5) and Barristan was not, in fact, in attendance.