Is refusing to eat with one's mother late at night—in order to sleep early and wake up for Fajr prayer—considered disobedience to her, especially after the father's death and her feeling of loneliness?

Our advice is that you strive to convince your mother to hasten the preparation of food and to abandon staying up late, which causes one to miss the Fajr prayer. If that is not possible, then be keen to keep your mother company and to cheer her up by eating with her. This is not considered blameworthy staying up late, because blameworthy staying up late is that which has no benefit. Keeping one's mother company and cheering her up takes precedence over keeping others company. If sleep overtakes you for the Fajr prayer after staying up with your mother, you are not sinful if you fell asleep before the time of the obligatory prayer began.