I think I hear you saying you are angry with the daughter for using and somewhat abusing your friend? If the daughter only comes around when she can "get something" from her Mom, she sounds like a poor excuse for a daughter, but alas, she is the only one she has, so of course, she want's to believe her daughter truly cares about her. You see the truth and that would make me angry too.

I kind of feel like you would be punishing the mother, at this point. If you are being benevolent, and for that we always get some kind of payoff, then that's ok, I'd continue helping and try not to focus on what the daughter doesn't do. Focus more on the way you get to feel when being kind and helpful. If the reward from being benevolent doesn't out weigh the anger against the daughter, then I think you have to take a step back and not offer help.

You seem to enjoy the dog, so don't take it out on the dog. Just enjoy the dog. I hope this helps a bit..hard situation, I understand! Big hug for you.