My mother never fasted her missed days of RAMADAN. And now obviously she has so so many fasts left. What should she do?
She must make them up now.
She must estimate an appropriate number and add a couple more to be on the safe side and make them all up. She can start fasting the 3 white days of the lunar month and Mondays and Thursdays with double intention of missed fasts as well as the Sunnah fasts and get double reward.