My friend is on ''the bran flake diet''?

she was 11 stone and is now 7.7 stone..she has been on this diet for 6-8 weeks! She is highly reccommending this diet to others but im not going to do it. I am already a healthy weight. I am worried about her. She is getting thinner everyday i see her at school. I told my mother. She said that the Bran Flakes shlould not have that effect on you. She eats 2 small bowls a day with some fruit in between. Is it the bran flakes that is making my friend loose so much weight? Please tell the truth!