Bismihi ta'ala
As long as she is under the age of "ya`s" or shar'i menopause, then she must observe 'iddah, which for her is not three menstrual cycles, but rather three full months.
In fiqh, a woman who is in the age of menstruating, but does not menstruate, she is referred to as mustarabah. Of course, it would be a good idea for her to refer to the specific view of her Marja' taqleed.
And Allah knows best.