Actually, I think FFC 'skipped over' the aftermath of Fredo's death because 1) it was absolutely irrelevant to the story, 2) to include it would have COMPLETELY ruined the impact of the ending of GFII, and 3) he most likely did not foresee that a bunch of nuts wondering about it on a yet to be invented internet keyboard some 30 years later.

Apple

ps - Personally, after that embrace at their mother's wake I think only 2 people on earth knew of Fredo's eventual demise: Michael Corleone and Al Neri. We could even go out on a limb and guess that Tom had suspicions just from being with the Family all his life and knowing Michael so well. But that's still speculation since as we see he is in no way involved with the killing.

IMAAAAAAAAAGINATION !!!!