It really boils down to the fact that Katie is not actually overweight, although she is near the top of the scale for healthy weight.
http://www.bhf.org.uk/bmi/BMI_Calc.html
She has no need whatsoever to follow any diet plans, etc. Just needs to keep an eye on her weight, excercise and eat healthily. What she is doing is just not good for her and may cause her long term health problems.
Maintaining a healthy weight is for life. Very overweight people probably do need dietary advice from a qualified dietician.
Perhaps Katie ought to see her GP for proper advice.