"Well, it's a good thing that Mr. Greene is usually found in the Underworld bar, where Jordan keeps everybody smiling and pleasant, because who wouldn't want to paste a tabloid reporter a good one? It's hard to get the warm fuzzies off a guy who feeds off the ills of society. You're in the business of trying to put terrorism out of business, and you get the feeling that terrorism is the kind of thing that keeps him in business.
The inescapable impression surrounding Greene, is that if you find him alone in the bar, it's only because he's already talked to everyone there, and he's just waiting for the shift to change. Sometimes with good old Joe you might get the feeling that while you're pumping him for information, he's watching you a bit closer than he should. Maybe it's your reflective glasses putting him off. Maybe it's the raw magnetism of your augmented abilities. Maybe."
Joe Greene has written numerous articles for the Midnight Sun. You can read some of them on that page.
If you talk to Greene and ask him for information in early parts of the game, Manderley will chastise you for talking to the press.
If you speak to Greene in the Free Clinic after being asked to kill him and mention Stanton Dowd, after meeting Dowd you will immediately be attacked by a single thug and a quite large force of MJ12 Troopers, thus further proving Filben's claim that he is a spy. Strangely, this happens even if you kill Greene immediately after talking to him. Something happens even after asking about Dowd to Shea, as the (neutral) thug will follow you on distance, acting like nothing's happening.
Greene can be killed at any point in the game (though if you kill him in the bar, you must be careful not to be seen with an unholstered weapon); if killed before returning to New York to find Stanton Dowd, Filben simply won't mention him and you won't get the objective, and Manderley will reprimand you for killing a civilian (if on the first visit, in the bar).
It is possible for Sandra Renton to mention "a guy in a grey jacket" claiming to be your cousin that is looking for you; she suspects he is a spy. It is likely that this was Joe Greene, as few other named characters in New York wear grey.