Pain during menses is normal if its not very severe. It is known as dysmenorrhea and occurs in every female. The intensity varies more because pain threshold is different in every individual.
You should worry only if she has history of severe pain
which hampers her daily activities and is not relieved with medication. Also if the pain is associated with heavy bleeding, passage of clots then it indicates some disease of the pelvic system. She can use mefenamic acid
and NSAID's to relieve the pain when present.
If pain gets severe or she has heavy bleeding she can get an ultrasound
to rule out pelvic pathology.