What do you advise me to tell my mother, who was unable to fast for years due to pregnancy, breastfeeding, and illness, and does not remember the number of days she missed, and feels distressed about it?

If your mother is able to fast, even if at a later time, she must make up the missed days of Ramadan. The obligation to make up days does not lapse with the passage of time; it only lapses in the case of permanent inability, in which case she must feed one poor person for each day. If she does not know the exact number of days she missed, she should make up the number of days she is certain will clear her conscience, which is the most scrupulous approach, or the number of days she most likely believes will clear her conscience. If she is unsure between two numbers of days she missed, she should make up the larger number.

In addition to making up the fasts, she is also liable for an expiation for delay for each day she delayed making up the fast without a valid excuse, while knowing that delaying is forbidden. This expiation is one mudd of food. However, if the delay was due to a continuous excuse or ignorance of the impermissibility of delaying, then she is not liable for this expiation.