Fredo would still have been banished and killed eventually, but I think Michael would have treated him differently. He would thank him for the information and order him to stay on the compound and not use the phone or other means to contact anyone until the hearings were over. He would have also appointed Neri to keep a close eye on Fredo.

He would then tell Tom about the situation and they would have basically hatched a similar plan. Tom would go to Pentangeli and tell him his brother would be coming to the U.S. to watch him testify, and then lead him down the road to suicide, at which point Michael would have done the same thing at the hearing (which in fact might have ben cancelled on Pentangeli's death since there would no longer be a perjury trap).

Thereafter Michael would send Neri and Fredo on a fishing trip.