I am in agreement with Steve here. Without the friendship, there is no opportunity to undo the underlying damage.
Sorry, but you can't break out of the friend-zone by being a friend. That's pretty much the worst possible advice. It's the one way that is guaranteed to fail. Niceness isn't going to win this battle; it only seals your fate.
Oh and withholding emotional considerations doesn't mean you're unfriendly or cutting off all opportunities; that's black and white thinking.
I am in agreement with Steve here. Without the friendship, there is no opportunity to undo the underlying damage. Forget about whether she feels hurt by polygamy! Focus on the fact that she thinks she has "discovered" that Christianity is wrongheaded, and undermine the reasons that form the basis of her unbelief. Keep the friendship going as long as possible, unless or until she fornicates with another man, and guide her back to the truth. Without that faith in God that she once had, she cannot accept polygamy as anything other than an archaic principle held by ancient people who didn't know any better.
It depends on what one thinks the root cause of the marital problems is in this situation.
Unbelievers have successful marriages all the time. Her no longer being Christian doesn't make her suddenly want to leave him. Doesn't make her suddenly disrespect him or despise him. What's the root cause?
I don't have enough information to say.
What caused her to be disrespectful of him? What caused her to no longer be attracted to him? What caused her to not see him as her spiritual leader? How did he end up in the friendzone?