This happened to my mother and sister under similar circumstances.
Fresh meat and especially fibrous veggies are much harder to digest than carbs. Also, I think carbohydrates(sweetness) trigger more salivation that would be missing in a low carb diet.
New foods are going to be harder to digest because her digestive enzyme concentrations aren't yet adapted to tackle more "complete" proteins.
1500-2000 kcal is probably too much.
My mother had problems with 1000 kcal of veggies and meat to start out with even though she had no problems downing a daily 2000-3000 kcal of garbage foods previously.
She eats 2500 kcal now with 40-50 grams carbs.
As a reference for you, she's lost 20 lbs of BW so far looking for 10-20 more.