different anon here, just giving her two cents. i think it's less that Lafayette was an Assassin and more than Connor trusted him as much as he once trusted Washington so he didn't have a problem showing him things like the armory or telling him of his plans to kill Lee. and of course, because Lee was acting unusual and going directly against orders, Lafayette would have been happy to help since he and Washington were pretty close and Lee's actions were detrimental to Washington.

i think there's potential for him to be a recruit or something, but i don't think he ever actually was in the Brotherhood during the game. that's why he was confused when he saw the paintings up on the wall. that's why he would have been content with just going with Washington's decision to let Lee live. he didn't know of the Templars so he didn't know that Connor was hunting them down and he didn't know that it was one of the main goals of the Assassins to kill Lee.