Answer:
(Fatwa: 356/356/M=1431)
If your mother verbally did not make a vow for keeping fast, so simply due to intention keeping fast was not wajib, and when it was not wajib so there is no kaffarah (expiation).
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
