As far as what to ask the Pastor.

1. What approach do you reccomend they take with their D?
2. I am afraid that this may end up upsetting my wife and driving her away, what can we do to mitigate that?
3. Are there any books you can reccomend I read on the subject of affairs and marriage therapy?

I think those are good for a start...basically tap into your apprehensions and voice them on paper..then approach him with those concreate defined concerns and see what he has to say.