I just read a previous thread on this topic...I think BOTH Fredo and Rocco were the traitors. Incidentally, it's clear from many scenes in GF2 that Fredo and Rocco are good friends. Check out the scene where Rocco gives Fredo a "love pat" on the face after telling him, "Michael says if you don't take care of this I have to." (Talking about Fredo's wife making a scene) Also, just as Michael is walking into the boathouse to question Fredo about the Senate investigation, Rocco walks out of the room...he has obviously been talking to Fredo.
I think Johnny Ola first contacted Fredo with the plot against Michael. Then Fredo supplied the players for the hit (Pentageli and Rocco). I think Fredo made Johnny and Roth promise that he (Fredo) would not be anywhere around when the hit went down. (Fredo's "You guys lied to me." Johnny's "Your brother won't find out.)