I suppose this might be a tough one, but this community has proven to be able to come up with good answers far beyond my imagination, so I'll give it a try.
Bob and Alice have been married for two years, now, and they have a small child together. Alice had (and still has) an affair and a couple of one nights stands. Bob found out (about the affair), Alice wants to get separated and divorced. They're both practising Christians and Bob refuses to accept their separation and divorce for religious reasons.
Bob argues that is against God's will that they be divorced. Unless he (i.e. Bob) actively releases her from that marriage, he is not allowed to have sex with another woman - even if Alice would give him explicit "permission" (which she does). He is using this to put pressure on her implicitly stating that he will suffer until the end of his life because of her. Of course he also refuses to officially "release" her because he is convinced that it his obligation to bring her back on the right path instead of (him) just running away. His only options are to continue to live in marriage or - as he calls it - war! There's nothing in between! He is aware that their child will likely suffer from that, but that's only because it's parents are separated and it's mother refuses to do God's will. He keeps quoting the Bible and searching for "qualified papers" that support his position ignoring anything else. Anyone not explicitly sharing his point of you is at least an idiot.
What I'm looking for
My goal is to help my friends settle their differences in a peaceful way so that they both can be happy and provide a nice upbringing for their child. At the moment, my friendship to Alice is much better and I'm afraid she's the only one willing to listen to me and actually trying things I may advice. However, I'm always open for suggestions on how to approach Bob, although I'm convinced that this will be really, really hard.
I will give you some more background information - read as far as you think is necessary.
What caused their separation
After about two years of marriage things between them got troublesome. Most of my knowledge is from Alice who talks quite openly to me about anything. I have no reason to believe she's (intentionally) lying to me but I am aware that her position is - of course - biased. Knowing both of them I find most of what she's saying at least highly plausible.
Alice felt neglected by Bob. It's true, he has a very demanding job, but he prefers to "relax" a few hours a day in front of his computer rather than spending some time with his wife (or child). They had sex (obviously), but less than once a month and to him it was more like a "let's get over it" thing. He neither seemed to really enjoy it nor was concerned about whether his wife did so.
Bob is in his early fourties (like me) and he has been one of my best friend for more than twenty years! In our twenties we both had relationships that lasted few years. We were both engaged - and we both separated before getting married or having kids. My separation was mostly consensual - it simply didn't work - while he got cheated on. We're both Christians and we spent a lot of time discussing religious topics always in an "academic" style - we were not praying together, for example. Up to a year ago or so I thought the way we think about "being Christian" were not equal, but at least similar. I tend to be a bit more liberal, he's sometimes slightly more conservative and "dedicated", but that's about it.
Bob is very, very self-confident. What he says is right and no one should dare challenge his position. (It's not that he's getting angry, though, but rather ignorant.) This goes as far as saying a bunch of psychiatrists are idiots unwilling to diagnose Alice with borderline personality disorder - which he is convinced she has - for reasons he can't really explain. He very rarely makes mistakes. Further, admitting having made a mistake and perhaps even apologizing is even more rare. It is my impression that he is truly convinced he's right and always has been rather than secretly knowing he's wrong but not admitting it.
Alice is almost 20 years younger than Bob, very good looking with a "troublesome" personality. She has been (and still is) in psychiatric treatmend for anxiety disorder and bipolar disorder. Regardless, they got married within a year, moved out of town together and now have a child. I have become a very good friend of Alice and at the moment my friendship to her is even stronger while I feel somewhat "alienated" when it comes to Bob.
During her youth, Alice was quite uptight about sex - partly because a some "incident" in the past. This is when she got to know Bob. It seems she's over it, now, and she loves and enjoys sex more than anything else. She really wants to try things, experiment... You get the point. Bob was open for none of those things so she began looking for it somewhere else and started an affair - which she is still in - plus a couple of one night stands. Of course Bob found out and as a consequence she began to force a separation and things got dirty. Bob was calling Alice a slut on just about every occasion - even in front of her parents - while Alice made Bob move out of the apartment. While Alice feels guilty she is convinced that she had absolutely no alternative other than doing what she did unless she wanted to "make her whole life a complete and utter misery". Bob is absolutely certain that he has done nothing wrong and even if there was some slight mistake on his side, his wife has to tolerate that and do her "duties" in their marriage - which of course includes monogamy and obedience when it comes to his (sexual) needs. (He's not assaulting her, though.)
Further updates (This is where I may add things that happened in the meantime if I think they might be important)
Bob announced that even if Alice returns to him he won't ever even touch her again, because she is "unclean" now and will be for the rest of her life. If she doesn't return, he will do his very best to make Alice's life as miserable as he can.