in Greg's defence (spoilers)
If you were to make this film from Greg's point of view, Seth could very easily be cast as the villain. Greg gets him the job there, takes him under his wing, and Seth repays him by cosying up to Vin Diesel, to the point where he puts Greg's job on the line by doing his own recco because he wanted to win Vin Diesel's approval; and worst of all getting with Greg's ex, which is borderline behaviour at the best of times, never mind when Greg specifically asked him to stay away from her. Twice.
And then to cap it all Seth turns rat and gets the entire company taken down.
tldr - Seth broke the bro code bigtime. I am on Greg's team.