Just try to be friends and keep expectations low. She is like a drug or alcohol, and unfortunately if he has to come in contact with her it will be difficult for him. I don't think he really wants *her*, but the body chemicals/hormones the affair caused are probably difficult to resist. Just remember it's not HER, it's the feelings that whole thing created... those are difficult to put aside.
Do you remember being a teenager and you'd like a guy and your body chemicals would just go nuts? You'd crave that feeling and think you were in love with that person. And then, after awhile, you'd come down from that "high" and wonder what you ever saw in that person anyway? And then someone else would inspire those feelings. I remember realizing that it wasn't the guys I was "in love with," but the feeling I got!
I think that's what these affairs are. It really isn't about the person (a particular OW just happens to be a person available and open to the situation), it's just body chemicals it's like drugs or alcohol and I think we have to hope our husbands can be stronger, and try inspire strength and character and "feed" their hormones and emotions in positive ways (if it's negative or belitting the lure of the body chemical addiction will only be stronger, anything to avoid the negative painful feelings of having hurt or destroyed someone truly loved...).
Anyhow, when you look at it that way, you can sort of understand how difficult it could be. I cannot predict the future of my marriage. I can only try to keep myself physically and emotionally healthy and try to be positive for me and my family regardless of any individual struggles the people in my family might be dealing with.
There is no arriving, ever. It is all a continual becoming.