The first time Michael meets Geary is at Anthony's first communion, and that very night is the attempt on Michael's life. He leaves immediately, so there would never have been a time when Fredo could have told Michael
that Geary was a regular at his whore house.
Geary's statement to Tom re. the dead hooker: "We done it be-FOE-er," tells me that he was a regular at that brothel with that whore. Given his prominence as a US Senator, the manager likely would have taken due note of him the first time he showed up, and reported it to Fredo. First reaction: Fredo was too dumb to report Geary's presence at his brothel to Michael. But Fredo was just simple enough to see it as something that'd give him bragging rights. Remember how he bursts in on Michael while he's with Sonny's daughter Francesca and Gardiner Shaw, shouting, "Hey Mike, guess who's here? Frankie Five Angels!" I see him bursting in on Michael the first time he learned of Geary's peccadillos: "Hey Mikey, guess who's a regular at my joint? Senator Geary!"
