I understand your concerns. I had the same ones as did the others here who have decided to give their spouses another chance. Ultimately you don't know he won't do it again. There are no guarantees in life. It's up to you to decide if you want to give him and your M another chance because it's what you want to do. And know what your dealbreakers are moving forward. Then if you give him a chance and if he has another A or continues the last one you can move forward on your own knowing that you gave it your all.
It's a tough decision. It took me a month of really considering it before deciding to give BF another chance.
I don't know how some LBS's deal with their spouse still working with OP. In my sitch the OW was laid off during the height of their A (not related). If she was still in that office I would have insisted that he get another job if he wanted a chance with me. I know it's easier said than done but I know personally that I wouldn't be able to deal with it.
There isn't any grounds they can terminate her for? The company doesn't have a morals clause or fraternization policy?
If you love somebody, set them free. http://tinyurl.com/2empx2g