What is the ruling on using water obtained through illicit means, whether for drinking, ablution, or watering trees and eating their fruits? And what should be done to rectify this situation, given the difficulty of convincing the father? And does charity expiate the sin?

The father's act of taking water is a forbidden aggression, and he must desist from it and estimate the amount of water consumed, then pay it to the responsible authority, or to public utilities if that is not possible. It is not permissible to take from this stolen water to irrigate trees or to drink, and whoever knows its condition and takes from it is like an usurper. As for the fruit, there is no harm for you in benefiting from it after the trees have been irrigated with that water, but you must guarantee the value of the water you used for irrigation. If the mother has paid money to the mosque with the intention of absolving herself from the sin of stealing water, and the amount equals the value of the water and cannot be delivered to the responsible authority, then you are absolved of the sin by that. As for the people who use the water without knowing about your father's theft, there is no harm upon them, and the sin falls upon your father.