Logically:
--If Roth wanted to have Michael killed in his bedroom, and...
--the drapes had to be opened so the gunmen could have a clear shot at Michael, and...
--a security guard was bribed or planted to open the drapes, then...
What would they need Fredo for?
I think that it's a very reasonable inference that Fredo opened the drapes.
When that morphs into Fredo must have opened the drapes, and from there Fredo must have known it was going to be a hit, and then Fredo must have been promised to be boss, then I think the inference breaks down. It ignores that Fredo says he didn't know, that Michael says he knows Fredo didn't know, that a generation later Kay doesn't seem to have resentment that Fredo tried to kill her. It also undermines the real tragedy of the film, which is that Mike's own rigidity is leading him to murder his own brother - over a screwup.
TB, you raise a good point.
We know from the late-night phone call that Ola is using Fredo for information. ("Will he come alone?")
I'll speculate that they needed Fredo for something similar in Tahoe. Details about the security arrangements, ways in and out of the compound, etc.
Or maybe Fredo opened the drapes. Obviously, that's a possibility. I happen to think it's not the most likely scenario, but it might have happened.
But, as Pete says, it's a bit much to build all kinds of scenarios on guesswork about a sequence in the film that, when scrutinized carefully, doesn't entirely make sense.
I also agree with Pete that the subsequent actions of the characters argue against Fredo's culpability.
To his list, I'll add Michael's poignant confession to Cardinal Lamberto. It's hard to imagine Michael being so wracked with guilt over ordering the death of someone who not only came within a whisker of killing Michael, but also Kay and, potentially, Mary and Anthony. Nor would Michael describe an assassination attempt as "He injured me."
