there is a medical condition called amenohrrea (pardon my spelling) which is the lack of perior for six months in a row. She too can be irregular. If she's overweight this is quite normal. She should go to the doctor and have it tested. It migt just be lack of a hormone or vitamin. If she is sexually active she might have a possibility to be pregnant.
"What does it mean if I skip a period?
There are several reasons why you might skip a period one month:
Age - If you have just started having your period, it might not come every month.
If you are an older woman who is approaching menopause, (when your periods stop), you might also skip a period.
Exercise - Excessive exercise might cause you to skip a period.
Stress - Stress and highly emotional times in your life may cause you to miss a period or two.
Illness - If you are sick, you might also miss a period.
If you miss more than a period or two (if you have been having regular periods previously), contact your health care provider."