Oli and LW:

Logic is on your side. There is no logical reason for Roth to have confided anything to Fredo other than what was required for him to betray Michael. But what we're shown and hear in II indicates otherwise:

--Fredo got out of Cuba and to NY in the same short timeframe that Michael got out and went to Vegas to meet with Tom, Rocco and Neri. Michael had a private plane, Fredo didn't.

--Despite being in Nevada the entire time Michael was away, Tom knew Roth had a stroke, recovered, got out of Cuba on a boat; the bodyguard was dead,and Fredo was in NY.

--Fredo knew the Feds had Pentangeli--and that Questadt belonged to Roth.

What that tells me is that Fredo told Tom about Roth's escape and the bodyguard; and that Fredo escaped Cuba on the same boat that rescued Roth. My conclusion is that he was in with Roth far deeper than he let on--for whatever (illogical) reason.

As for Fredo seeing Questadt with Roth:

Oli and LW: you probably recall an astute catch made by someone on this board: he spotted Questadt sitting behind Roth in Batista's meeting with US businessmen. Someone else posted one of many earlier scripts in which Questadt and Michael met in Cuba. It was dropped from the final cut, leading to the assumption that either FFC overlooked Questadt in that scene, or wanted to save money by not reshooting it (it was filmed in the Dominican Republic) and hoping nobody would notice Questadt in the scene. But: If Fredo had spotted Questadt with Roth, so would have Michael--in the same room with him.

It's immaterial because the Questadt/Michael meeting wasn't in the final cut. That's why I'm convinced thatFredo must have learned about Questadt from Roth or Ola.