I think a lot can be deduced about Fredo's involvement in the plot against his brother by examing not Fredo's actions after the hit attempt, but instead Hyman Roth's and Johnny Ola's. The simple fact the Johnny Ola plays along with Fredo's act like they never knew each other says to me that he felt a certain sense of responsibility for misleading Fredo.

I think that if Fredo had suddenly acted like he didnt know a hit was planned after the hit failed even if he did have prior knowledge, then Ola and Roth would have more openly protested against his cutting of all ties with them.

Since Ola plays along quietly, I believe he had misled Fredo.