Should i just take the abused dog or get the police involved?

ok there is this mutt (not sure what it is) that has found itsway to my parents house. There is a lot strays and abused animals around our house so my mom keeps dog food and cat food out at all times. Her owner knows shes there but doesnt come and get her. Shes been at my moms for about 2 months now. She goes home one day a week, but doesnt stay long. Shes had several litters, you can see her bones even though she eats. And needs alittle medical care. I think with lots of love she will be the perfect addition to our family. She is the happiest most loveable dog ive ever met. Im thinking i should just take her. She lives with my parents anyway. But i would also be stealing a mans dog and i dont want that to be heald against me on judgement day. The dog looks pretty rough and if i call the police and get the shelter involved she might be put down with out ever being given a chance. Or they would take her and i would have to adopt her Costing more money, and i live 100 miles a way. And i dont want to get involved in any animal abuse cases. He has lots of other dogs that he takes really good care of. He just doesnt take care of her. the mans a druggie and drinks alot so im not sure if its safe to go to his house and ask to take his dog. Ill probably call the police but was wondering what would you do?