I think it would be harder to forgive or justify because the affair went on for six months. A one night stand, might be seen as a really bad mistake (although I personally could never forgive this), but I think 6 months is thought out deception.
But then again, it is your friends life and decision, and I think all you can do is be there for her whatever she decides and whatever the consquences come out of it.
I had to sit and watch my sister take back her husband (although then, he was her fiance) after he had a four year affair with a girl. My sister had a son to him, then when my sister got pregnant with her second child, the split up for a week, in which he had a one night stand with the other lady again, and she in turn became pregnant. My sister STILL took him back. All we could do as a family was stand by her. They appear to be happy now, 5 years later, but its her life.
Sorry for the ramble.