I should have been more clear in my post. I'm not trying to prove the fact that he's guilty, but the extent of his guilt.

Fredo's knowledge about Pentangeli and Questadt implies he was in contact with Roth throughout much of the film. And if this is the case, then it can only mean that Fredo was determined to remove his brother from power each and every step of the way, NOT just the botched Tahoe attempt.