Well (maybe I'm wrong here, and if I am, sorry), in my opinion, it's not so much the number of calories you consume than it is about WHAT you consume. I mean, I'm barely topping 100 lbs, but I don't consider myself healthy--most of what I eat is loaded with sugar. I'm not necessarily saying your mom's diet idea was 3/4 sugar--and hey, if she lost some weight, more power to her!--but I'm saying that most of the time, what you eat is more important than how much you eat.
Basically, you could be healthier with a 2,500 cal. diet of fresh fruits and vegetables than on a 1,500 cal. diet of junk food.