While the Mt. Washington Valley region is a rather large area geographically, the population is quite small and people travel around quite a lot -- Conway/North Conway to Lincoln, Glen, Jackson, Gorham, Littleton, etc. and even over into Maine.
When we're in the mountains we usually stay in Bartlett but it's not unusual for us to drive all the way to Jackson for dinner at a good restaurant, or stay in Lincoln and drive over to North Conway for shopping. The locals all know each other because they drive to work over the same routes. There are only a few high schools in the region so everybody either went to school with each other or played against each other in sports.
So it wouldn't surprise me if NK knew Abby from having run into her at a restaurant or store or sporting event or church, or knew her parents, or knew friends of the family, or was related to one of her girlfriends or teachers, or while out hiking or skiing or hunting or whatever.