Something else that hasn't been mentioned, maybe the neighbor is this girls father. He may have had an affair 16 or 17 or whatever years ago and fathered a child and just doesn't want to tell anyone. It is just as likely as many of the scenarios that have been thrown out there.
Wouldn't THAT be interesting? Of course, this isn't Days of our Lives, so unlikley.
I'm sure that over the years, there have been threads on "should I tell a friend that her husband may be cheating?" Let's say you see your friend's husband out at a restaurant, being cozy with another woman. You don't have all the facts, but it still nags at you. Did I see something I shouldn't have seen? Should I tell my friend? Most of us end up minding our own business, but it's difficult to be in that position, and difficult to get it out of your mind.