The finger points at Fredo. In the later boathouse scene, he admitted that he betrayed Michael by giving Johnny Ola "information" about the "big deal" Michael and Roth were negotiating. That's BS: Fredo wasn't in on the deal--what "information" could he have given Ola? His only value would have been to open the drapes. But, that would have been incredibly risky--how would he have known that Kay was asleep? And, how could he be sure he wouldn't wake her?
I don't think the word "information" is used. Fredo talks about "a little help":
JOHNNY OLA bumped into me in Beverly Hills -- and he said that he wanted to talk -- he said that you and -- and -- ROTH were in on a -- a -- big deal together. And that there was something in it for me if I'd help 'em out. He said that -- he said that -- you were bein' tough on the negotiations. But if they could get a little help -- and close the deal fast -- it'd be good for the family.
But yes, opening the drapes could be a little help, if it would helpful to know where Michael was at a given moment.
Coppola and Puzo were in a tight spot: Fredo had to do something for Roth that would trigger Michael, but that wouldn't trigger the audience against Fredo. The kidnapping notion of early scripts obviously wouldn't resolve that dilemma. I think their final choice was just to punt the question - leave it a mystery and let the acting of Pacino and Cazale carry the day.
