My mother made a vow for a particular need or wish that she will read two nawafil daily throughout her life. However, as she is aging and sometimes gets involved with different things, she is not able to perform them daily. Is there any kaffarah for that? She is really worried and anxious because she performs her obligatory prayers on time but misses out on these nawafil.
If she did not enact a vow to perform two supererogatory prayers (nawafil) daily, and instead, merely thought to herself that she would pray two nawafil every day, then there is no obligation on her to perform them. Enacting a valid vow requires a formula to be recited, which includes the name of God (e.g., if such and such happens it is incumbent upon me to pray two supererogatory prayers daily for the sake of God).