Your body can scavenge from dietary protein and fat to supply glucose. But the hoops your body has to jump through to do that requires more effort than depleting your body's stores of glycogen. Glycogen is converted to glucose rather easily. When this happens, your basal metabolic rate drops because you have less lean muscle tissue burning calories and your body thinks its starving and cuts back on energy requirements. The human body's genetic material is 12 million years old and still thinks in terms of feast or famine.
Protein is used to build and repair lean muscle tissue.
This would not provide enough glucose to prevent ketosis. Also, since most sources of protein also have fat, I would guess that a diet that included 500 grams of protein would also inherently contain at least 214 grams of fat. Proteins are large molecules and you blood is constantly filtered by your kidneys. Twenty grams of carbohydrate is only 80 calories. I would bet that a lot of the weight your wife has lost is water because each gram of glycogen in muscles and lean tissue holds 3 grams of water.