Ummm...reality check - this is definitely about infidelity. (I wonder if her husband knows anything about this?)
Probably retirement-related depression and late midlife crisis. Is he taking any medications that might be affecting his mood or judgment? He probably started an online conversation with this woman that turned into an emotional affair.
I understand you want to be understanding and sensitive to the fact that he may be going through a crisis, but also be careful of getting yourself into a position where you are tolerating intolerable behavior. (If MY ex husband had told me he just "had" to go spend a week having an affair with another woman, I would have told him "fine, but don't bother to come back. I'll ship your things").
