Prenups get tossed out for very concrete reasons, like fraud, duress, lack of mental capacity, ridiculous terms, lack of legal representation etc. If a prenup is all valid and simply says that both parties will take away whatever they brought into the marriage, it will not get tossed out. To claim that a prenup will get tossed out simply because the husband acted like a teenager and anonymously blogged about the cheating wife is paranoid, in the politest term. I will stop practicing law if his prenup gets tossed out just because he blogged about his cheating wife.
For libel, there has to be no factual basis, and you need to publicly reveal the person's identity. There is no libel here. She cheated and he didn't give her real name or picture.
The lesson here is not to keep your mouth shut about your cheating wife and be a sheep about it.
As for your question, I'm not interested in the moral side of this man's actions. Personally, I think he is a whiny bitch. But legally speaking, this will have either no bearing in the courtroom, or it will have a positive outcome. In fact, the only legal consequence of adultery in Western law is divorce terms. Judges rule slightly in favor of the party who got cheated on when it comes to dividing assets and deciding alimony. So if this blog will be taken as evidence at all, it will either be in his favor or will have no effect at all.