Her blood sugar may not be stable enough because her body is still too used to carbs and sugar. Once the body gets used to more fat and protein it burns energy differently. Insulin will be lower. When insulin is low the body can flip into beta-oxidation and burn fat really well as a source of energy, providing the body with the fuel it needs. I'm sure Mark's book explains this more, and Good Calories, Bad Calories by Gary Taubes explains this really well in the last few chapters in case you want to read more about it. Many here recommend having dinner at night, and then pushing breakfast out until noon or after (I guess some call that lunch). That's a good 18 hour fast right there. That block of time is easiest for me to fast, because I spend a lot of it sleeping. Good luck!