If I had to take sides, I'd definitely be on the side of the betrayed wife here. But, I don't think anything is that black and white. What about forgiveness? OrmIrian hasn't been betrayed herself by the husband. She is friends with both of them. And we don't know what their marriage is like. Their marriage may have had serious problems before this affair, and while he's taking the worst decision of having an affair instead of trying to fix things, if we knew his side of the story, we may understand more. It's possible that he's just a twat, but taking OrmIrian's character assessment of him at face value, he sounds like a basically decent guy who's made a huge, huge mistake (and I am not condoning his behaviour and underestimating the pain he has caused).
I think it's possible to be there for someone without ever condoning what they have done. If the husband is genuinely sorry for what he's done, then he's more likely to be able to take the long, hard road towards making everything right if he has some understanding and forgiveness from his support network. Understanding and sympathy is not the same thing as condoning his behaviour.
Abedelia says it's about a year before the betrayed party in a marriage tends to start getting over it. Sometimes it can take longer, I'm sure. Like custardo said, it's the emotional betrayal that cuts deepest in many cases, and it's far harder to get past that than knowing your OH had sex with someone else. He has to let his wife go through this process while accepting that he is the cause of all this pain and that he has to wait for her to be ready to move on at a point that suits her, not him. Absolutely. But imagine living like that. Feeling like shit for 12 months or more, and knowing that it's no one's fault but your own. That's not easy, and while it may be deserved, many people will just throw in the towel because it's easier to start again than to achieve forgiveness after a very long time of being told that everything is your fault. Guilt is one of the most destructive emotions around and if it's not addressed, it can lead to serious mental health issues. If he really wants to save his marriage, he has to be in a fit mental state to do that. It takes grit, determination, a lot of self awareness and huge comittment to make a marriage work after an affair and that requires the husband to have people he can talk to about his feelings. He's made an awful mistake but he's still a human being and should not be denied that.
I know I'm going against the grain here, and I stress that this is my opinion and therefore isn't necessarily the best course of action, but in OrmIrian's position, I would be there for both of them. I think it may actually help the wife if the husband has someone to talk to who reminds him why he has to go through the guilt/earning forgiveness stage and why it will be worth it in the long run.
My only point of caution is to watch what you say. Never say anything to the one party that you wouldn't be happy to have the other know about, otherwise you could end up losing both of them as friends. If you think that's likely, you might be better off staying away until the first ramifications have passed.