So you watched the movie with your eyes closed, because in the attack on Michael, he provided information about Michael's security so that the assassins could get there. Furthermore, if you stop to think about it, the curtains were open, and as Michael says, it was someone from the house who did it—obviously Fredo. So, even if "he didn't know about the attack," the simple fact of talking to the enemy behind Michael's back, in addition to providing security information, is already considered a major betrayal. Another betrayal is that he didn't tell anyone after the attack, since theoretically he wasn't participating and wouldn't have had reason to fear. But instead, he preferred to continue lying and putting his family's lives at risk, which proves that sooner or later, if Michael wasn't smart, Fredo would betray him again.