Thanks for writing to DoctorSpring.com with your health concern.
STress, anxiety, emotional turbulence, late nights, travelling, erratic eating, smoking, alcohol, ingestion of hormonal medications ( birth control or emergency contraceptives ) , exercise, weight changes - all this can delay periods. Is any of this applicable in her case ?
Mild hormonal imbalance also leads to delayed periods.
IF the pregnancy tests are negative, then please consult a gynecologist.
Go for internal exam- per speculum and per vaginum and pelvic ultrasound scan.
Also hormonal levels - FSH, LH, TSH, Prolactin, DHEAS, Free testosterone.
This will reveal any condition such as polycystic ovaries, endometrial hyperplasia or hypothyroidism if it exists.
All the best & Please feel free to discuss further.