The first period probably wasn't actually the begining of her cycle.
It is quite normal for a girl to have what is known as a stray period and then have nothing for up to a year, It works the other way around at the other end of your menstrual life.
When you are hitting menopause
you can have months and months without and then suddenly it will appear again.
Tell your cousin that she is quite normal.Had this happened a year or so into her periods it would be a different story but in her case it isn't at all unusual.