Tom seems a little reluctant to give Michael information about Fredo. First he dodges the question altogether by talking about Roth, and only when Michael becomes insistent does Tom say that Fredo is in New York. This suggests to me that perhaps Fredo and Tom had been in some kind of comunication and Tom was keeping this from Michael. In that scene Tom already seems to be aware that Fredo was in on the treachery because Michael says something to the effect that Tom is to tell Fredo h knows that Roth misled him. This leads me to believe that Tom suspected that Michael would kill Fredo for his betayal, and in his typical fashion was already trying to broker a deal. I could easily see a terrified Fredo calling Tom asking for help, and Tom saying that he should lay low until Michael returns and that he would talk to Michael. In any case when Michael seems to be in a forgiving mood, Tom has no problem contacting Fredo.

Assuming Fredo was led to believe that Michael thought he had been duped by Ola and Roth, he could come back to the compound and live in relative comfort and safety. For Michael, as I said earlier, I think it was simply a case of "keeping his enemies closer." Obviously Fredo would have been striped of any role he previously had in the family business. The thing is that Michael may well have tried to de brief Fredo, as did Tom, but Fredo stonewalled. In fact Fredo knew something of Roth's plan B cause he knew about Questadt, and he never disclosed this to anyone until it was almost too late.