My friends dad mistreats his dog

Okay so I've got a friend who I've been friends with for at least 5 years. His dad brought a german shepard puppy about 2 years ago. I am certain that his dad and sadly maybe even my friend mistreat the dog. I'm 100% certain that the dog is never walked. His dad is fat, lazy and has major health problems and hes only like 45. He's just lost his 2nd kidney, so obviously got a new one. He eats nothing but garbage and drinks shit loads of fizzy. I believe the only reason he got the dog was to be a guard dog. The dog is also super aggressive and I know 3 other people with german shepards, which are all super friendly. The dog bite my hand once when I startled it and went to pat it, even though it had been super friendly with me about 2 days before hand when I was throwing her the ball. The dog violently barks at me if its behind a gate, door or on a lead, but it ignores you if it has free access to you. Like once I had to walk up my mates long drive way by myself and the dog had been let out and it just followed me and I just ignored it and kept my hands away.

The other thing I wanna talk about is I think my mates dad kicks her. I remember my mate use to say that his dad doesn't treat the dog well. I also heard my mates girlfriend say not long ago that "she only listens to you if you kick her, but I'm the only one who doesn't kick her". I knew as soon as I meet the dog that its been hit though, without anyone having to mention it, as I've noticed it cower when my mate tells her to "fuck off" and puts his hand in the air as a threat or walks close to the dog showing dominance.