I agree the R he has with OW is not healthy. As for why it is lasting this long, it just may be because they feel like they have a pact, an us against all of them mentality. I hate to say this, but it seems like until they start to feel they no longer need to fight for there R, they may start to focus on the R and see it for what it is.
As for moving on, I keep reminding myself about "acting as if" with that situation. I am in the same place as you, where I know I do not and can not move on, but I know that if I keep pushing, I will get the adverse effects of what I am trying to acheive. I believe that if he send him the letter or confront him now, it will only stand to remind him of what he has given up for the OW and make him work even harder at that R to convince not only you, but himself that it was worth it.
