Roth probably had been invited to Anthony's party. He begged off, citing ill health. But he knew that Pentangeli would be there, contentious over Michael's support of the Rosato brothers. Thus Frankie would have a perfect motivation to whack Michael, and would make the perfect patsy if the murder had succeeded.
So, what information did Fredo give Ola and Roth? And why did he protest, on the phone, "You guys lied to me"? A guess is that he confirmed that Frankie was invited, and planned to be there. Another (speculated by others on this board) is that he gave Ola the layout of the grounds and the location of Michael's bedroom window, and opened the drapes.

What was the "lie"? That's a toughie. Some here have speculated that Ola told Fredo that Michael might merely be kidnapped and held hostage until he agreed to Roth's (unspecified) demands. That idea gets reinforcement when Fredo said, "I swear to God, Mike, I didn't know it was gonna be a hit." It's credible that Fredo's stupidity, coupled with his resentment of Michael, would make him fall for that absurd idea.

Another possibility: Fredo was told that Michael and Frankie would be whacked, and Frankie's body would be placed in such a way to look as if he'd killed Michael and had been killed in return (perhaps by those two gunmen out of NY who'd been found later). Then Fredo would take over as Don. Thus, the "lie" was that Frankie had been permitted to leave the compound before the attempt on Michael's life, that Michael survived--and that Fredo wasn't the Instant Don.

Of course Roth had intended for Michael to be killed. He didn't have to have Frankie killed at the same time, because suspicion would fall on Frankie no matter where he was. And, had Michael been killed, Roth would have little more use for Fredo. If Fredo succeeded in becoming Don, his coglioni would always be in Roth's pocket. If a power struggle broke out over succession, so much the better--divide and conquer. And, with Frankie a hunted man, the Rosatos would take over New York. He couldn't miss. But when the murder failed, Roth had to go to Plan B--lure Michael to Cuba--and he needed Fredo to at least stay on board.