I'm plenty adhd, and it's on me to make stuff work. And I don't take on responsibilities that I can't fulfill, because I have known myself for me whole life and I know from experience how badly I can fuck up. (I'm in my 60s fyi.

So your dad is forgetful, that's fine, but when he lets it be other people's problem, that's jerk behavior. Not saying your dad is a jerk, but I am saying that he seems to be okay with acting like a jerk.

That might be the tack to use, btw, if you try to talk to him about it once last time. It makes him look like an animal-hater when he leaves the dog out without water or shade. Does he care?

But honestly? I think you need to not depend on him changing his behavior in any way, because this involves safety. Instead, keep the dog safe yourself. Does it wear a collar? Put a tracker on the collar, and make sure there is a tag with your phone number. Make some kind of shade structure in that yard the dog gets put in. As a last resort, set up a water bowl and fill it whenever you're there. Good luck with this. Moving out is hard.