Most of the weight would be from water and lean tissue. I'm not sure what your point is. If all she ate every day was the equivalent calories of refined white sugar, she'd lose weight, too. Probably more than the butter, actually, since all that sugar would probably send her thyroid through the roof. Both are bad ideas.
Fat isn't very anabolic. Out of fat, protein and carbohydrate, a diet high in fat will likely lead to the most lean tissue loss than anything else. A diet high in protein and roughly even in fat and carbohydrate (but still a calorie deficit) will likely preserve the most lean tissue. Exclusionary dieting in a deficit (low fat or low carb) seems to perform worse in studies than a balanced approach. And, of course, the more aggressive the deficit, the higher you want your protein to be and the more you should lift weights to preserve lean mass.