We're all human, we're all fallible, there are so many shades of grey in between black and white.
I think the reason why is far more important than the actual cheating. Were they just selfish arseholes only ever interested in self gratification? Were they in a horrible marriage that they didn't have the strength to leave? Were they as a couple going through a really bad patch and the affair becomes a wake up call? Did one partner already absent themselves from the emotional side of the marriage and left the other one on their own? Are they both good people that just married the wrong people? Did they change a lot since getting married and meeting someone else brought that out?
And so, so many other reasons.
I know of a few couples where they guy was married to their work, absolute workaholics. In a few the husband had the affairs (partly due to ease from travelling, partly due to emotional connection to someone they worked closely with, in a couple of them though the wife had the affair because she was so lonely. But all of them were caused by the emotionally and physical absenting of the husband.
As a friend, not my position to judge, and if I was friends with the wife then in either circumstance I would be there to support them.
If they were selfish arseholes that were happy to cheat whenever they felt like it, then I wouldn't want to be friends with them because that selfish nature would inevitably come out within the friendship as well and I wouldn't want that in my own life anyway.