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.
I really don't think Fredo was
that stupid to betray Michael
again. I think he was too relieved that Mike had forgiven him and that he was allowed to return. Fredo would always be in the position to do damage to Michael, but I think he wouldn't have tried it again. Especially if Michael kept him out of the family business.