Amazon sells them. They are an ultrasonic sound device that humans can't hear. I tried one, but it wasn't powerful enough to penetrate the neighbors garage to bother the dog. It might work for you. If you do talk to them, say only you are worried their dog is in deep mental pain and leave it at that. Show compassion for the suffering pup and hope for the best.
@Bess Barber Here in Canada, bark control devices can be found at most all pet retails, hardware stores and some livestock retails, also at the vets. Sometimes you may have to special order.
We had a real problem with dogs barking at our old house. The people across the street had two dogs, one big and one little. They'd put one outside on the fenced patio and it would bark unceasingly while the other one would bark inside at the patio door. WOOF-WOOF-WOOF...yip-yip-yip. Then they'd switch dogs and it would be YIP-YIP-YIP...woof-woof-woof. We'd call the cops. The cops would come. Everything would be fine for a few days and then it would start up again. No barking dogs in the new neighborhood, thank goodness.