Asking her why she friended OM - what answer could she possibly deliver that would make it appropriate at this critical CRITICAL point in their marriage? What's the point in asking?
The point in asking is to get information and try to understand the other person's point of view. It says to the other person, "I care about you and your feelings. I hope that you also care about me and my feelings." As long as both people approach each other will hardened feelings and closed minds, there will be no improvement in the marriage.
