High-quality plain greek yogurt is actually relatively low-sugar and packs a lot of protein; the stuff I buy is 140 cals/serving, 23g/protein and carbs under 9g (carb number might be lower but not higher than that, I don't think...would have to look, not at home near the fridge atm).
If someone's going extremely low-carb then yogurt in general is not a good option, but it's a good source of calcium and protein.