What can I do about this dog?

My mom and I live with my uncle and my cousin and we have for about 6 months now. For the last 4 months I have been feeding their dog as well as ours. They feed her crappy food and what they do is just fill up two steel bowls and leave it in there and check on it after a few days and pour some more in. Well, she hates this food because she doesn't eat it unless it's during the night and she just nibbles on it b/c it's the only thing in her bowl until I give her food the next morning. Now, I feed mine Purina but now that I have a job I'm switching them over to Blue Buffalo as soon as I get my first paycheck. Also, I know to swap them over slowly. My concern is their dog. I want to feed her Blue Buff too. I treat her as though she's my dog b/c I basically take care of her. So, naturally since I'm switching my dogs to a better food I want to switch her too. But I know they're going to keep feeding her the junk they're feeding her...I can't tell them not too. So, if I go ahead and feed her Blue Buff but they also feed her their food, will it make her sick to eat such a high quality food and also a low quality food? Should I just keep feeding her Purina and not feed her Blue Buff to avoid making her sick or do you think she would do ok if I go ahead and feed her Blue Buff too?