I think fruits diet with natural nutritional supplements is fine.
The body needs other nutrients besides what fruit gives us. With a fruit diet there is basically no protein (minimal protein in fruit) or fat intake. You will lose weight; however, the body will eventually become deficient of protein and fatty acids. Recommend a balanced diet with all nutrients - protein, carbohydrates, fat, vitamins and minerals for a healthy body. You can lose weight slower, however, in a healthy way. Thank you for your question.