That page 2 illustration had me completely confused. Looks like her/his back does not even exist. Also why does MC refer to the devil as "her" and "she" when its a boy?
I'm thinking a lonely end would be sorta fitting for this. The guy can't move on from his wife, and it's not like the co-worker can just slide in that easily because key part is the MC himself. He clearly does not love the co-worker and has no reason to.
At this point, it seems he loves his wife so much that he can't consider other girls as an interest and might move on from love(romance) altogether. Possibly a lonely end(Possibly hinted at a while ago)? Not that being lonely is particularly bad, but maybe he would be able to accomplish what...