This is a normal blood pressure. Your mother must be thin and lean. People with low body weight tend to have low normal values of blood pressures which are also beneficial in the long run.
As a matter of fact your mother is not actually “suffering” at all. There is no such diagnosis called “low blood pressure” in outpatient setting in Medicine. But some practitioners brand a set of patients as having “low blood pressure” and suggest high salt diet which is completely rubbish.
Next Steps
If your mother has any of the symptoms mentioned in the next section, seek medical help
Health Tips
If the BP of an individual is lower than required, there WILL be symptoms of low blood flow to organs like
1. Decreased passage of urine
2. Giddiness on getting up from lying down posture
3. Episodes of fainting
4. Cold fingers & toes when compared to the body temperature, etc.