Is a 1200-1400 calorie diet to low?

My mom is 144 lbs and 5'2". She was on the South Beach diet, but didn't lose any weight. She switched to a 1200-1400 calorie diet, but she tries to stay around 1200. I said I thought that was too low and it should be around 1500 at the least. She doesn't believe me and says she doesn't even think she is losing weight on the 1200 to 1400 calorie diet. I've heard of people dying from malnutrition when counting calories, how many is a safe diet?