I quote from the first of @Ailia‘s sources:
“Dr. Robert O. Young’s New Biology™, most simply stated, is that the over-acidification of the body is the single underlying cause of all disease.
In contrast, the old biology, based on the work of Louis Pasteur in the late 1800s, stems from the idea that disease comes from germs which invade the body from the outside.”
BTW, the above was written by Dr. Young himself. He is not a medical doctor at all, and has been charged two third-degree felony counts of practicing medicine without a license (plea-bargained down to a reduced misdemeanor charge).
His dietary advice relies on live blood analysis. Live blood analysis lacks scientific foundation, and has been described as a fraudulent means of convincing patients to buy dietary supplements and a medically useless “money-making scheme”.
This article summarizes the scientific lack of validity for the alkaline diet model.
As to the second, whether you accept the idea of food becoming ash in the body, and health relying on acid-base balance or not, it does not say don’t eat fruit. It says “our diet should consist of at least 70 to 80 percent basic foods – that is, no more than 20 to 30 percent acidifying foods,” fruit being one of the acidifying foods. In other words, do eat fruit, but also eat other foods.