What is the legal ruling concerning the questioner's father, after a traffic accident that caused the death of a person, and he was found to be 25% at fault, keeping in mind that he is elderly?

If your father's percentage of error in the accident is known, then he is only liable for that percentage of the blood money, and his 'aqilah (male relatives on his father's side) will bear it. He also owes an expiation, which is the freeing of a believing slave. If he cannot find one, then he must fast for two consecutive months. If he is unable to do that, then nothing is required of him. And everyone who participated in destroying an inviolable soul must free a slave, because the expiation is indivisible.