So sorry to hear this and what rude neighbours who don't take care of their dog. I was thinking of one thing concerning the flea chew, though; Castor has a tick prevention that I think works not as a repellent but by killing off any ticks or fleas that land on him. Can it perhaps be the same with your dogs' chew, i.e. that it would have killed the fleas within a couple of hours or so? That might explain why your dogs got fleas although they had taken their meds. Castor got hedgehog fleas a few weeks ago (which would not have stayed on him anyway as they apparently don't like dogs) so I now know how absolutely awful it is to see fleas on your dog's coat.
Good luck with getting rid of them!