IF they are into reading/researching, here are a few of my bookmarked suggestions, but pinpointing a specific resource isn't going to be the magical link.
They need to get with a good trainer(you?), IMO as well as doing it on their own.
Nothing in Life is Free
which may produce results immediately
Shut up and walk away: The art of Loose Leash Walking
All My Blogs | Roger Abrantes
he has many articles worth reading
tab289 channel - YouTube
Paws Abilities | Helping people enjoy their dogs.
Dr. P's Dog Training: Modal Theory