something just doesn't smell right. I don't think they were doing it, because what good would he be to her (or himself) if he was face down on the couch. I would be very suspicious, especially with a history of abuse in the realtionship, as the article suggested.
If they were sleeping in bed, I could see her rolling on top of him and him not able to move and accidentally suffocating him, but sitting on a couch, presumable watching TV. Even if they were play fighting, he would have been thrashing around as soon as he couldn't breath. I'm guessing it probably takes someone a good minute or two before they're dead from lack of oxygen, and during that time, he would literally be fighting for his life.
Something's not right here.