Another possibility: Fredo was told that Michael and Frankie would be whacked, and Frankie's body would be placed in such a way to look as if he'd killed Michael and had been killed in return (perhaps by those two gunmen out of NY who'd been found later). Then Fredo would take over as Don. Thus, the "lie" was that Frankie had been permitted to leave the compound before the attempt on Michael's life, that Michael survived--and that Fredo wasn't the Instant Don.
Roth had to go to Plan B--lure Michael to Cuba--and he needed Fredo to at least stay on board.
IMHO it is more likely that Fredo DID know it was going to be a hit and he lied to Michael when confronted (just like Carlo tried to do). I also agree that Roth or Ola led Fredo to believe that Pentangeli, or someone would be set up as a patsy, and that no one but Fredo would ever know what happened. But, as Michael put it, the assassination attempt failed, and the asssins were killed, as Michael said, by "somebody close to us...inside..very, very scared they botched it." It is also noteworthy when Michael and Tom run down the list of potential suspects, Michael makes the point that Rocco and Neri are "businessmen" but that Fredo is "weak and stupid."
By the time Ola calls Fredo, Michael had left Tahoe suddenly and without explanation; Tom was temporarily in charge of everything including Fredo and his men; Geary was about to be compromised; and the identity of the mastermind of the hit was still considered a mystery. Fredo, with a guilty conscience anyway, was now rightfully afraid that Michael would find him out. In short, Fredo now knows it is possible that Michael will win. Ola tries to reassure him while simultaneously using him as a mole to inform him whether or not Pentangeli is on the level in making the deal with the Rosatos. Fredo truthfully says he doesn't know anything, and then tellingly says that "you got me in deep enough already."
I don't think Roth "lured" Michael to Cuba, I think it was Michael's plan to go to Cuba all along. The Cuba deal was the big thing Roth and Michael had going and the reason Michael initially denied Pentangeli permission to welch on Clemenza's deal with the Rosatos. Had Michael been assassinated, and Pentangeli set up as the killer, Roth would have reached out to Tom and Fredo and no doubt convinced them that this new investment would garner more strength and money for the Corleones in the wake of Michael's tragic death. Further such an arrangement would forever protect Fredo from being found out by Tom, Neri or Rocco.
I can hear Lee Strausberg laying it on. "Michael's death?...
a tragedy. I always thought my time would come before his, and that is why I had planned to leave everything to him. Now, I am keeping the offer I made to Michael ... and am giving it to you." They would have both bought into that, and of course they would have been Roth's targets in Havana insetad of Michael.
