I keep asking, how? Michael exiled him from the Family. He was only allowed cursory contact with his mother. He was banned from the Corleone hotels.
Before the betrayal, Fredo obviously wasn't a part of the business, and knew nothing about the Cuba deal. But Johnny Ola told him about the Cuba deal, and found in Fredo a willing partner--because Fredo had a grudge against Michael. Banished or not, he still had a grudge, which would have deepened with banishment. Anyone who knew him would know that--and might find out something of value from Fredo because he'd be willing to seek revenge--again.
