FWIW I agree with this, even if my last post read differently...
I think kids should ideally be a priority and something you work for. But only if the relationship is feasible to begin with.
This wife of the OP's hypothetical (or actual situation) cares more about her newfound affair than having a stable family. In that situation, divorce is ideal, simply because the relationship will not work, even if another situation (nuclear family, or heck, an open relationship where the hypothetical husbond is fine with it) would be ideal.
While kids are important, and that sacrifices should ideally be made for those kids, if the sacrifices aren't coming from the heart, they're not worth it, then everyone will be unhappy.
The wife in the OP doesn't care enough about her husbond or her kid to end an affair. This is a premise for divorce: because that's all she values her husbond.
I personally have experience with open relationships and think it's great, much better than normal relationships. But I wouldn't end my current closed relationship because of that; it's too much of a personal sacrifice to lose her. I value her much more than my ability to boink random chicks or guys out in town.
The op's wife, however...
(Well, I'm sure she'll be able to parent well even if divorced. This is not intended as woman-blaming)
The point is that she does not like the husbond's idea of relationship anymore, and he doesn't like hers, so they should probably end it.
God damn it I'm rambling a lot right now.