Except when does the movie ever say or even hint that Michael would be kidnapped? What would the purpose of that even be? Fredo would know exactly what he was doing when opening the drapes.
It was in a draft of the script (second revision, I believe). When confronted, Fredo does his "I didn't know it was gonna be a hit!" routine but he says something that he was told by Ola that it was going to be a kidnapping; I guess the idea would be to grab Michael, force him to work out a deal, and release him (basically like what Sollozzo did to Tom in I).
