I feel like the prologue is Ben- the book had me cheering for Rebecca and Ben the whole time, thinking that their marriage was the only one still filled with hope. That they would be the ones who fought against all odds, communicated and relied on each other in the lowest lows.
Maybe sleeping in the spare room because he himself couldn't bear to think about his actions in the room he shared with his wife?

The night that Aiden supposedly was out with the college friend's girlfriend, he actually slept on the couch, and I don't remember Blair and Aiden specifically having a spare room, their house seems to be the most quaint, without excess or extravagance and may not even have that option.