We will never know what Fredo did or didn't know or do re. the Tahoe shooting. But let's go with what we do know:
1. Fredo actively conspired with Roth and Ola against his brother's interests, for his own personal gain. ("Johnny Ola bumped into me in Beverly Hills...he said you were being tough on negotiations...but if they could get some help, there'd be something in it for me."[emphasis added.]
2. Fredo's involvement with Roth and Ola was deeper than his disingenuous "bumping into" Ola in Beverly Hills. (At the Superman show: "Johnny Ola brought me here...old man Roth would never come here, but ol' Johnny knows these places like the back of his hand.") Fredo could not possibly have gone clubbing with "ol' Johnny" during that visit to Havana, right under Michael's nose. He had to have been to Cuba with Ola (and maybe Roth) earlier.
3. Fredo had expected to be named head of the family. He was deeply resentful of Michael. ("I'm your older brother, Mike...I was passed over...IT AIN'T THE WAY I WANTED IT!!!)
4. The drapes in Michael and Kay's bedroom were opened to give the gunmen clear shots at Michael. ("Michael, why are the drapes open?" followed by machine gun fire through the window.)
5. Fredo had some role in the events of that night. ("You guys lied to me"--to Ola on the phone afterward; "I swear to God, Mike, I didn't know it was gonna be a hit"--to Michael in the boathouse.)

The last two statements could suggest that Fredo didn't know his involvement would be fatal to Michael (and maybe Kay). But, it is a fact that Fredo betrayed his brother for his own gain; and that Michael and Kay were nearly killed as a result.
The best-case scenario (and it's a reach, IMO) is that Fredo was so terminally stupid that he thought that Ola and Roth might want to kidnap Michael and somehow convince him, without harming him, to let Fredo take over. Anyone stupid enough to fall for that is a danger on and on. Worst case (and the one I believe): Fredo knew good and well that he was opening the drapes to facilitate his brother's assassination, or simply closed his eyes and ears to that outcome in his lust to take over the family--and for revenge. Either way, he couldn't be forgiven or trusted.
Yes, DC, Michael was definitely a cold-hearted bastard. And he acted cold-heartely in the way he lured his brother back into the bosom of the family, tricking Anthony and Connie (and Fredo) into believing that he forgave him. He could have had him whacked in New York (Tom evidently knew where he was, as in "Get word to Fredo...") and spared the rest of the family that cruel charade. But Fredo deserved to die--he was a danger to Michael and his family.