If she is dead-set on breeding her, chances are that there won't be much you can say in terms of getting her to change her mind. Have you asked her for what purpose she plans to breed? If the answer is anything close to just having puppies, I'm afraid there's not much you can do or say. If that's the case, the health, temperament, breed-ability of the female (or even the male), possible stillborns or c-section or other complications, veterinarian care, lifetime support, possible return of puppies from buyers, etc etc. is not at the forefront of their purpose...it's simply to produce puppies. Someone who has it in their mind that it would be great to have puppies, doesn't usually want to hear the negative talk...essentially and unfortunately, they may have to experience raising a litter firsthand in order to understand the enormous responsibility that comes with it. I hope you can find a solution that satisfies both you and your mother, and the dog isn't the one who suffers as a result.