A sad situation that seems to be mirrored in many parts of France with dogs and also with cats. Many of the cats that end up with us have obviously been handled and yet at 6 weeks seem to be cast out to fend for themselves. That is like dumping a two year old child on the streets.
It's a tricky situation .. you want to have the problem resolved but the dog owners don't seem to perceive there is a problem. Plus you don't want to anger the owners who might take their anger out on the dog. Have you talked to your other neighbours ? Perhaps see if they have any suggestions ?
It is true that many French people, especially in the countryside, regard animals in a very different light to we Brits. I found this out when we were living in a rural gite right at the beginning of our stay in France.