I'm a dietitian.
Protein is 4 calories per gram.
Carbs are also 4 calories per gram.
Fat is 9 calories per gram.
The calories in a protein shake is determined by quantity and what you've mixed it with. If your protein powder recommends 20g scoop, that will be 80 calories if mixed with water, usually a few more as there's also flavourings etc. It will be even more if you mix it with milk. Most tend to be 30-35g, which is 120-130 calories.
Protein is used to build and repair muscle. Muscle gets torn during physical activity, protein helps repair that. To build muscle, people will generally consume protein after a workout, and people working out a lot, will need to consume more protein than the average person. Most women will need 45g of protein per day. That's a large chicken breast, half a steak. Vegans will often take a vegan protein to ensure they reach that goal, but most meat eaters do not need protein powders unless they're looking to do serious muscle building and looking to consume 1g of protein per lb of bodyweight.,
Someone of Demi's size needs to prioritise dropping weight and not thinking about building muscle. She shouldn't even be looking at a protein powder. At her weight, cutting 250-500calories a day should be no issue, like, just take a scoop of those giant plates of takeaway. Doing that every day should be enough to see a difference in no time.
To the person skipping breakfast to "claw back calories", do not do this. It's a recipe for disaster. It creates an association of punishment, so you're depriving yourself of breakfast, which makes you eventually want to rebel or feel more hungry and makes you more likely to eat more later and creates that diet feedback loop. Thing of those takeaways as part of life, you needed to rest, or you were celebrating. Doing that every single week is what causes long-term problems, but if you think on a long-term basis, not weekly, it's fine. Think per month- this was one week. People binge one day on a diet, then write off the whole diet because they were "bad" or "couldn't do it". I always tell my clients that you wouldn't quit your job after one bad day at work, you'd just shrug it off as a bad day and turn up tomorrow.