I don't know GF2 as well as others, but to me it is ambiguous as to what Fredo knew about Pentangeli, and when. Tom tells Michael about Frankie, cites the New York detectives as his source, and says that he thinks Fredo doesn't know anything. Fredo does then tell Michael about Pentangeli and Questadt, but there's no way to know how or when he found out about those things.

One thing that seems obvious is that information about Pentangeli became much easier to obtain once the senator announced that a surprise witness was coming. That's natural, as prople with information might open up once they thought the trap had been set up. So it could be that Tom and Fredo were both picking up the same information at the same time.