Some women have much shorter cycles, lasting only 22 days, and some have much longer ones, lasting up to 36 days. The average menstrual cycle or gap between the periods is 28 to 30 days, but different women have different cycles, and this is perfectly normal. It depends on symptoms she is experiencing before, during or after menses, weight gain,  difference in the amount of discharge during menses, etc. consider consulting a qualified physician. I suggest to get her thyroid profile and ultrasound pelvic organs done to know if she had any problem. Physician may guide you in case she requires any other tests.