You will loose weight like my friend did, then two months later he is back where he started, he also felt sick and weak.

Is going on an apple and water diet healthy?

I wont ramble on with the usual – we both know anything beyond a good diet and exercise will only offer a temporary solution – your body is where it is at due to your eating and exercise habits – the body adapts to the new diet by using fuel reserves (fat) and muscle tissue – you essentially weaken the fat burning furnace and slow your metabolism (it will be starving, and therefore new fuel will be taken and stored as a reserve/fat)

When you resume your normal eating and exercise habits you will adapt again similar to how you are now, perhaps 2lbs lighter, perhaps 2lbs heavier, but almost certainley slightly more body fat due to the stress of the above diet.

lose weight and keep it off is to eat less fatty foods

The only healthy way to lose weight and keep it off is to eat less fatty foods, smaller portions, and a variety of healthy foods and excercise. As soon as you get sick of apple and water you will gain it all back.

Apples are basically just carbohydrates… so you’d be getting no protein and no polyunsaturated fat/essential fatty acids. You also wouldn’t be getting the wide range of vitamins and minerals you need to function properly.

Apples are basically just carbohydrates

No amount of pills and whatnot can replace real food… because things you can take in pill form are not biochemically the same as real whole food. A lot of vitamins/minerals rely on each other for absorption and you wouldn’t be consuming them in the good ratios that are in the actual foods.

The only way she could get any protein is by having protein shakes, which I don’t think she’s doing because that wouldn’t be just apples & water. Also by having no fat, you won’t be able to absorb any fat soluble vitamins. It would lead to general protein & fat deficiency, fat soluble vitamin deficiency among other things.