My .2 cents is that its not the zero calorie butter. Even though I agree that its crap and I personally would rather find a way to include the real stuff in my diet.
Have you recalculated her BMR at the lower weight? Lets say its gone down to 1450 roughly and given that eating at 1200 calories is just a guess, if you are over by a little bit here or there you could easily be eating at maintenance over a week.
Sometimes people just plateau, wouldn't hurt to do a refeed one day either. Then go back and drop calories to 1100 for a little while.
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