Sometimes women just miss a period for no reason at all. There are also other medical reasons (not pregnancy) that may have caused her to miss it.
The blood test will give you a definite answer.
She WAS on birthcontrol? Does that mean she stopped taking it and is no longer on it? IF so, that is a perfect reason for a 'late' period. I went off my birth control in September, never got my period, October, never got my period and November never got my period BUT ovulated and became pregnant so I went until December without a period and started getting symptoms so I tested and I was in fact pregnant. After getting off bc it can mess up your cycles and take up to a year to get them normal again.