My mother in law won't listen to me about my dog.?

My mother in law lives with me and feeds my dog almost anything. He is only a year old and she will feed him an entire bag of treats in less than an hour. As well she continuously gives him things like stew, chili, hotdogs and anything she might be eating at the time. I have no problem giving him table scraps but he throws up all the time now, especially when she's been feeding him. She refuses to listen and laughs it off because none of the other dogs get sick and she is convinced that it's because he has a habit of eating plastic. The only thing is even when he hasn't eaten anything foreign for weeks she still blames it on that. I'm afraid he's going to get diabetes like their other dog did, what can I do to get her to stop?