I have found also that taking the dog and getting it to listen to YOU so proving to the owner that their dog can do it with the right guidance can assist in helping them get over the whole 'I've tried everything and my dog will never get any better' attitude. Obviously you need to do this with the owners consent. But if you ask kindly if you can have a go then perhaps the light-bulb moment will occur.
"In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog." - Edward Hoagland