Is it really true you cannot detect ovarian cancer with ultrasound? I heard of someone who had one and it found tumor.

NO. It is not true, it is easy relatively inexpensive safe test , with no radiation exposure.
Untrue. A good pelvic ultrasound by an experienced ultrasonographer with up to date equipment should be able to detect most ovarian cancers unless it is extremely small. Laparoscopy would be suggested if a cancer is suspected, therby the ovaries can be carfully inspected. Final diagnosis depends upon a pathological tissue diagnosis.
Ultrasound helps. Diagnosis needs biopsy/resection of the lesion and examination of the tissue by a pathologist. Ultrasound can reveal a mass in the ovary, that would prompt biopsy/resection. Thus ultrasound helps in making the diagnosis.