Re: Is it me...am I over-reacting? Honest opinions please
Sounds like you're both being defensive.
It's hard to glean any kind of real story about what's going on from all this, but if he's still browsing her facebook page (by the way, how do you know? because it does sound like an invasion of privacy for you to check up on that) then he's still thinking about her. But that doesn't mean he wants to start anything.
Sometimes when you care about someone, even if you know they're no good for you, you want to help them. The way you've described the past situation (long phone conversations) implies to me that he cares about how she feels. In itself, I don't see a problem with that. I can't say whether it went further, or whether he wanted some kind of romantic or sexual relationship. She might have, but it isn't her intentions that matter—it's his.
My advice in pretty much every situation like this is to have a good talk, and try and ask questions in an honest attempt to understand. And try and word your sentences carefully so it's a safe conversation to have, and neither of you feels judged or un-cared-for. Pick a neutral environment, and try to assume the best of the situation throughout.