I don't think Fredo would have agreed to or aided in a kidnapping, even HE wasn't that dumb. For one thing, I think he knew his 'kid' brother well enough to know that a kidnapping was the LAST thing that would get Michael to agree to anything Roth proposed. It would only anger him more.
Obviously Fredo helped in some way, but as he himself admitted, he was pretty much kept in the dark.
I don't think Fredo opened the drapes.
I don't think he killed the gunment either (sniveling coward that he was)...but that's another thread.

.
Apple