That's how Cowboys fans are. They're bandwagon jumpers. Back in 2005, when the Cowboys were doing really bad and the Mavericks were really really good. You couldn't find a Cowboys fan in all of Dallas. The whole city was hog wild for the Mavs. There were even people that I know were die hard Cowboys fans claiming they've never liked the Cowboys. It's just ridiculous. That's what makes them so intolerable.
You're right about you're roommate. Had the Saints lost, you would've never heard the end of it. He'd still be talking about it 3 years from now. He was quiet in case they lost (which they did). But had they won, he'd rag you for years. I say don't let him off easy and give it to him good until the Saints and Cowboys play again cause that's what he would've done to you.