There is not a lot of research or studies on low blood pressure. The ninds.nih.gov site is a decent resource for health questions.
With low BP- it's up to the patient to tell the doctor when they feel it is off. Some tell tale signs (not limited to the following) are: dizziness
when getting up from a seated position, extreme fatigue
(all of a sudden, you become aware of the effort it takes to breathe), mental slowness/confusion and/or a general lack of energy/enthusiasm (flat emotion look to the face, sometimes with dark circles
under the eyes and a slight bluing of the face and/or hands).
One of the things to watch out for is a less than 30 point spread between the 2 numbers - that means the heart is not pumping well.
If she doesn't feel right then keep at the doctor or find a new one. I've gone through this with my parents and best friend - it's tough to find a doctor that will take it seriously but worth the effort in the long run.