From what I have been hearing on this board, affairs like this are short lived. So, don't presume he will be with her forever.
That being said, you have to decide how long you are interested in standing for your marriage.
Since this whole WAS thing has happened to me, I have a new perspective on why relationships fail. I used to think there were just a lot of guys who were jerks, or women who were romance addicts and that is why they left.
Now I think that it is more likely that most of the time one person simply became so unhappy in the R that they had to escape.
There is almost always two sides to each story.
Have you figured out if any of your behaviour contributed to the breakdown of this R ?
Maybe your H is just a jerk, but I do think that you have taken on too much responsiblity in the R and you became the housemaid and forgot to be the seductress.
Me 47, W 32,D 6, Met 11 yrs. ago, M 7 Bomb 4/08/08, Sep. 8/10/08, Div. 8/10/09