I would suspect that Fredo mentioned to Mike that he knew Geary from the brothel, and the planning went like this:

Meeting in Mike's office

Tom: This Senator....how are we going to deal with him?
Mike: Fredo says he frequents one of his "houses."
Tom: That's perfect for us. We can compromise him with this.
Mike: Compromising him isn't enough. Besides he'll be able to call in enough favors to keep this thing quiet.
Tom: What if we caught him and made it appear we were covering for him?
Mike: No, he's too slippery. The only solution is to silence him with something more. Al?
Neri: If he thought he'd killed one of the hookers....
Mike: Ok Al you get in there and drug him, and kill the girl. If Fredo is right he plays some kind of bondage game, so you make it look like he went too far.
Tom: He's got to think that you found out about what he did through Fredo.
Mike: I don't want Fredo directly involved. Al you do exactly what you plnned, and once the Senator is coming around I want Tom to go in there and convince him that we have the ability to keep this thing quiet. Of course you also have to convince him he's guilty.
Tom: That shouldn't be too difficult.
Mike: Good, but it has to happen after I leave for Havana. I don't want to be in Nevada when this happens.

(Problem is Tom and Mike forgot to tell Al to get th hell out of there, or at least keep himself hidden so the senator would not see him).