Treat the cause
I wish it were easier and simpler;however, as with an general symptom, many causes can lurk.
Not to scare, but a thorough psychiatric evaluation would be the best mother's day gift. Offer to go with her and to find one.
Most likely, this is psychiatric in nature : a form of anxiety
, compulsive, even phobia
(of death); but slim. Physical cause/contributing factor. We are complex beings.