Fasting for an Elderly Person with a History of Illness

There is an elderly person with a history of illness, currently not actually sick, but with general weakness. The doctor, who is not religious, recommended eating and drinking as usual without a clear explanation. Should he listen to him or drink in measurements? This is what he did on Yom Kippur and it passed without incident, thank G-d.

It is difficult to answer a question about the obligation of fasting without all the details. However, a patient who needs to drink or to eat on Tisha B'Av, may eat as usual and there is no need to eat or drink “by measurements”.

Shulchan Aruch, Orach Chaim,