Actually the family did know that Nick had been murdered by beheading a couple of days before we knew.
The reporter told them about the video tape.
That's why he collapsed in the yard.
Still, kind of odd--why would he collapse when a photographer just happened to be there--don't you think he would have collapsed 2 days earlier, and not wait until a reporter showed up? Or is the news of the existence of a videotape more unsettling than the news of the beheading of your son? I don't know.