Do you mean 5'1"?  At 5'11" and 100 pounds, she would have a BMI of 13.9.  I made it down to a BMI of 15 once when I was sick for an extended period of time, and that's absolutely miserable.  I wouldn't wish that on anyone. Edit:  I'm really hoping you didn't mean 100 kg, because that's obese no matter what a woman's height.