As upsetting as that letter was, you do have a responsibility to keep your dog quiet, even if it means extra work for you. You may not agree with or understand their point of view, but they are at least reaching out to you for a resolution.
My mother's neighbor sued her over her dog barking, and the legal ruling was that she had to either pay for her dog to be debarked or face removal of the dog by animal control...
Wow harsh. Why couldn't hiring a professional trainer be an option to atleast try? Was she warned several times or did this neighbor just jump into a lawsuit? Debarking is horrible.