True...but let's keep in mind that in addition to 'sweet & innocent', Fredo is also presented as 'weak & stupid'...not only by Michael's description but various instances throughout both films. Even in The Godfather Fredo makes one blunder after another. While it's true we find it shocking that it's Fredo who turns out to be the traitor...instances throughout the films show that he is just weak and stupid enough to be appealing to Roth as a dupe.

And yes, we do see in the final boathouse scene that Fredo was not so sweet and innocent that he wasn't capable of finally reeling off at his kid brother for whom he was stepped over.

But that still doesn't make valid the assumption that he realized there would be a hit and opened the drapes to make it so.

Apple