Silver said that the traitor was one of the " Legendary " Freedom Fighters. He's likely talking about the Knothole / New Mobotropolis Freedom Fighters who managed to defeat Dr. Robotnik / Dr. Eggman. Since he seems particular interested in members of that group (he openly accused Sonic and suspected Sally briefly in M:XYL) he seems to know that the Knothole Freedom Fighters are indeed the group he's looking for.

In addition he said that the destruction of this particular group brought about the ruin of his world. The Original Freedom Fighters were (presumably) destroyed when they were banished to the Zone of Silence ten years ago but nothing bad has come of it. In fact their banishment lead to the rise of the Knothole Freedom Fighters who did a world of good for Mobius.

That idea has been thrown around (in reference to the original FFs thing). Silver seems sure it's the FFs we know, and that certainly isn't a bad guess with their track record and team. Personally, I'd be more inclined to say that the traitor business is related to the current FFs in some way, it would seem a tad more fitting considering what we do know.

The one thing to take into consideration here is that it's Silver, and if Ian makes him anything like his game counterpart (and I'm sure he has), he's bound to have mixed up or mistaken some details, and it's possible he's being lead/tricked in the wrong direction (but still in or around the right timeline for when the "disaster" occurs). Hard to say at this point, we don't really have enough specific details to work with.