My father started this diet that his friend told him. Just drink one shake in morning, one shake at night.. and OK lunch at lunch time. But he's hungry, I know I would be too.

This diet is too limiting, is it OK if he eats something?

My father used to eat a lot and just drinking a shake is not going to fill him up at all.

If the diet is SlimFast (which is a powder shaken with 1% milk) that you take morning and lunch, they recommend eating balanced meal at dinner time and snacks in between.

Tick

This diet works by limiting calories. If he has one shake and two meals, it will still work, but not as quickly.

I would suggest he makes some vegetable soup and has this as well, as it will help to fill him up more without a lot of extra calories.

Are the shakes Slimfast or some sort of protein drink? If possible, go for the protein drink, they are much more filling than Slim fast.

You stay out of it and let your dad do what you dad wants to do. He is a adult and can make his own choices.

I dislike fad diets, and those that do not include a proper balanced diet and exercise, But his diet is his choice