Oh, poor dog.
Maybe you could talk to the neighbour and tell him about it, if you haven't done so yet? It seems like a horrible thing to happen to a dog.
From what I've read, a lot of pets feel lonely when their owners leave for work - and it's not something surprising, not when a person is forced to usually work for 8 hours per day and be away from their home.
Can you get to the dog? I know for sure that I'd pet the doggo. I once jumped over a fence to give my neighbour's German shepherd some water because it was summer, he had none and he was thirsty. Such a good and friendly dog. Unfortunately, he died last winter. Got scared by the fireworks and hung himself over the fence.