If the dog doesn't mind, I don't see any reason to worry. They are forming a bond in the way that seems to satisfy them both. When I was a girl, a stray kitten at my grandmother's farm started nursing on a stray dog that she has adopted, and the dog, who was spayed, actually started to lactate. At least my aunt believed she was lactating. Since it is true that even human men have been known to lactate if they nurse babies with enough perseverance, I think it could be possible. I know another stray kitten who chose an elderly cocker spaniel as her surrogate mother - not for nursing, but for sleeping with. It seemed to comfort the dog as much as the kitten. I guess love is love, the world around. Animals know how to find it.