Re comments about the fen country, if, for example, the Lake District is Coke, pop music or steak and chips, the fen country is a fine wine, a late Beethoven quartet or caviare. Its attractions are not immediately obvious but will grow on you in time. After 35 years I still feel nostalgic for the area with its huge skies and no annoying hills or mountains to spoil the view.
Re Chatteris, etc, just about everywhere I was posted in the RAF, people were commenting about the "funny locals", forgetting that they were "funny locals" from somewhere themselves! I agree with chopper's remarks about St Ives - I prefer it to our own tourist-clogged town of the same name!
If your friend keeps an open mind I am sure he and his family will have a great time.