thanks for writing to HCM.
your mother's problem can be related to soft tissue
inflammation usually at this age shoulder joint capsule (covering layer over the shoulder joint) can be inflammed if her shoulder movements are also restricted then this can be ADHESIVE CAPSULITIS
Also go for blood sugar test if it is raised then address the issue with your physician.
for the shoulder problem i would suggest you to take help of a physiotherapist nearby you.
He/She can provide your mother with some pain relieving treatment, shoulder mobilizations and some home exercises which will definitely help.
i hope this helps
take good care of your mother..