1200 calorie diet? too extreme or not?

Hi all. Anyone expert on diets here? I have a girlfriend (23 years old, 170 cm tall and has 70kg). She wants to lose weight very fast so she was thinking of a low calorie diet such as 1200 calorie diet. I told her that eating that little calories can get her in trouble but she won't listen to me as I am no expert. Do you think that 1200 calories is too low as well? Her BMR is somewhere around 1800 calories daily. For healthy dieting a 20% max calorie deficit is advised, am I right? Here's an example of a diet she wants to follow 1200 calorie diet. Although this diet looks like a balanced one, I am still concerned that she might hurt her metabolism and really slow down her weight loss process. What do you think? Do you have any other references I could show to her? (the downsides of too extreme dieting)