@celia16 "Anyone know if this can be fatal?" Short answer is yes. Here is a reference from Medical News Today:
Researchers from the University of South Australia have found a causal relationship between vitamin D deficiency and increased mortality risk.
Scientists also found a person’s risk of all-cause mortality increased by 25% if their vitamin D level was in the deficiency range.
https://www.medicalnewstoday.com/articles/low-vitamin-d-linked-to-increased-risk-of-premature-death-research-shows#:~:text=Low%20vitamin%20D%20linked%20to%20increased%20risk%20of%20premature%20death,D.&text=About%201%20billion%20people%20globally,was%20in%20the%20deficiency%20range.
Will your mother eat fish? Fatty fish would be a good source of vitamin D. Hopefully her doctor would advise the best approach.