Michael loses nothing by telling Fredo of Roth's plan to assassinate him.
his very anguished statement about the two of them not having spent time like this before. To me this i a dead giveaway that Fredo is the traitor, and it gives Michael the perfect chance to bring his enemies in closer by telling him about the asassination plot.
Now wait a minute. What if Fredo tells Roth that Michael knows? Then, any counterstroke Michael has planned goes out the window.
Also, I don't find anything in Fredo's "spent time" statement especially revealing to Michael. Michael knew that had kept Fredo at arms length for years, so it would be natural for Fredo to lament it.
