To take Forge late game, what may have worked would be using fully teched Spartanks, along with regular Scorpions, and then getting a view on Forge's reactors, double MACing each, and effectively cutting off his flow of Grizzlies/Wolves for a while. It would take a lot of resources to get back up to that point, and even if he has 10,000 at that point, at least it's like 30 seconds of no more reinforcements. You just would have had to act fast.
Other than that, yeah, having your buddy not go solo-Arby in the late game would have helped. Once you have a steady resource flow, the counter to the only guy you don't have covered is always a good idea. My former 2v2 partner and I figured out between us that you always want to have the Covenant player on your team go anti-vehicle.
Game on.