Anyone know about dog ownership and neglect?

I live in Oakland, California. My upstairs neighbor has a serious drinking problem and as of about Tuesday of last week just left his dog in the yard for two days. Eventually we let her into our apartment and gave her food and water. There's four of us living in our apartment and all of us have knocked on this guy's door trying to get him to let his dog back in but he won't open the door or acknowledge her. He's definitely home. In fact, I don't think he's left his place for a week. We're looking at adoption possibilities for the dog. Problem is, she's an old dog. Sweet natured and fine with other animals but around fifteen with bad hips. There are senior dog adoption centers but there's the issue of ownership since she doest technically still have an owner. One place said they would send me a basic template of surrendering dog ownership that we could try to get the owner to sign so that we get ownership and then can find a place for the dog.

What happens if our neighbor won't sign this paper and still won't take his dog back? Is there a point in time by which it can be considered abandonment and he has legally surrendered ownership? Could he sue us for giving his dog away if we did so without a signature from him?