She should try to figure out why it's low before going on Hormone Replacement Therapy. It could be the sign of an underlying health condition that can be addressed. For example has she been on a highly calorically restrictive diet? Thyroid issue? Problems with her ovaries? Is she taking any medications? Pituitary problem? Pregnant?
Additionally, she needs to check ALL of her hormone levels. Not just testosterone. And checked during the different phases of her menstrual cycle.
I would suggest that she see a doctor that will be more thorough. She should try to find a good endocrinologist.
But to answer your question, many women are on HRT. In fact, I would argue that nearly all post-menopausal women should be on HRT.