Since you are walking your dogs anyway, maybe dropping by with a new chain, a little bag of treats, new water bowl (doesn't have to be expensive, just clean) would be a great way to gain this lady's trust and show her you are really concerned. Offering to walk the dog would be nice, too. She may really appreciate a little help! You never know- with a little time, a little patience, you might be able to suggest in a friendly way she think about finding a better situation for the dog (especially as winter approaches!) But keep in mind, she may also be VERY attached to him, in spite of her lack of attention to his needs. Good luck with your efforts, your love of dogs really shines through!