HE DIDN'T KNOW, Michael deluded the weak-minded Fredo into believing he was accepted back into the family and that Michael had forgiven him. Look how comfortable Fredo is on the Tahoe estate later, interacting with Anthony etc., not the attitude of a man knowing he is about to be killed.

Fredo "praying too loud" was a device that Coppola with his brillant directoral vision so as to make the scene more dramatic than Neri simply shooting Fredo.

Who's to say Al didn't regulary fish with Fredo and Anthony? That would be Neri's mind at work, assisting his Don into luring Fredo into a false of security...