Logically, plawrence and afsaneh are right: Since Fredo previously told Johnny Ola that he was pissed off and wouldn't cooperate further, he'd have no logical incentive to call the Cuban authorities to rescue Roth. Logically, a dead Roth was a Roth who couldn't rat out Fredo's betrayal. But logic may have had nothing to do with Fredo's behavior. He was "weak and stupid," as Michael said earlier. A combination of panic and resentment of his brother may have pushed Fredo to try to rescue Roth.
I'm not in love with that theory, either--I just think it's possible. The Trilogy has many examples of stupid, weak people behaving illogically. Did Carlo really think that Michael wouldn't figure out the connection between his beating Connie and Sonny's assassination? Did Paulie Gatto think that no one would make a connection between his absence and the attempt on Vito's life?
Afsaneh: Very interesting point about Fredo possibly having realized his fatal slip of the tongue before the kiss of death. I doubt it, but it could be!