One of my brothers has a Golden Retriever like this. Timid as hell and afraid of everything. His wife and daughters set him up and bullied him into getting the pup. No research on breeding, etc., they had found this pup advertised for sale. It was older coming to them, the last from it's litter to find a home. Obvious to me that no one wanted this mutt, as they could see the timidity as a puppy.
They've had it for 2 years now. Same thing, different day. Close a kitchen cabinet door, it runs and hides. Slam a car door, it runs and hides. My setter goes to play with it and it screams, howls and carries on as if it's being killed.
A total piece of $h!t and there's no amount of time, patience, or whatever that's going to change the situation.
My sister had a rescue collie. A man would walk into the room and it would go hide. It finally started coming up to me at 12 years old. Total, neurotic piece of crap.
Sorry to be blunt, but face it, these people could have just such a dog.