I could say that your body is in starvation mode. When you are eating way below your BMR, your body sense that you are calorie deprive hence it will holds as much fat as it could; making your weight loss stagnant (this is the same situation as polar bear hibernation).

That's the reason why extreme dieting only works for a very short while and never produce any good result. First, you'll drain out the water in your body. Second, you'll have very low amount of protein hence your body will start to breakdown your muscle tissue (your fat burning factory) for protein, glucose, energy. Third, your body will retain as much fat as it could as it sense you are in starvation mode.

Well, Dr. Layne Norton do covers a lot about metabolic damage lately especially mid this year. I doubt he have anything to talk about starvation mode and metabolic damage if it's a myth.

SOS

Her smaller body doesn't require that much calories to maintain therefore the weight stalled or going too slow. Bones, organs, blood, water and skin has weight too. So if she wants to decrease further faster, either starve herself or sorry to say, remove organs or bones. 

Don't use scale that much. Ask her to use mirror, if she look puffy then cut out salt and sugar to prevent water retention. Eat more fiber that helps flush them out.

nah.. just joking on that part..
What i meant is that your gf's is already below ideal weight.. her progress will stall or go real slow.. i mean real sloooooooooooow even if she's at calorie deficit.. can you reduce the weight of your bones, organs, blood, skins, etc? idk man..
If you feel weak eating less then don't do so. Just eat at maintenance calorie, then workout to make it in deficit. How much you can cut depends on your training intensity. 

guys, its been 2 weeks since i started my diet. now, i feel weak and i think my body is smaller, im craving for food, i cant lift as i lift normally, so i reduce reps and sets just like  Dr. Layne Norton article mention by alien9 said.

my abs make a little progress, i can see it comparing last time almost none. i think my bf is like 15%-17%. its a good process by just 2 week.

so now, im wondering, how do u guys cutting and bulking, what do u do for your diets? im wondering while im diet, should i add more weight? or maintain?

before this im on calorie surplus. and it feels great, i can do many sets and reps. i feel stronger too. now im weak. makes me wonder if i eat more and exercise more but maintain by calorie deficit, will i achieved my goal(reduce bf)?

situation: eat by 2.5k cal and burns 500 cal through cardio. a total of 2k cal at the end of the day. will it works?

40kg x 22% = 8.8kg
38kg x 22% = 8.36kg

8.8kg - 8.36kg = Lost 0.44kg fat. How long does it take?  and those measurement might not be accurate. Female has higher bodyfat compare to mens.

Try lifting heavy, minimize cardio and increase protein intake.