She posted a day earlier in the week where she consumed 1020 calories for the whole day. The average woman who has a normal BMI would burn around 2000 calories a day, but being overweight you can burn hundreds, maybe even thousands extra. If, in theory, she burned 3500 calories a day and only ate 1020 that puts her in a deficit of nearly 2500 and they say it takes a 3500 deficit to lose one pound.
Eating so little isn't going to be sustainable for her at all. She said on her page this week she wanted to lose 3/4 pounds. In the first week or two it's definitely possible with water weight etc but after that if she's eating in that much of a deficit her metabolism will be shot and will make the rest of her journey all the more difficult.