Erm, we live in a 1 bedroom in nyc (manhattan) and I can assure you, most of us, take amazing care of our dogs.
With that said (had to get it out, my NY pride was hurt), you have vet bills proving you've taken care of him all this time. I would also get a dog license for him immediately if you haven't done so already.
Between the license and bills showing you've been supporting him, I think you'll be ok. Besides, there's really not much she can do since she did sell him.