From what I understand (as a nurse and having had menstrual irregularities) the only requirement, physically, is that the uterine lining be shed at least every 3 months. If it goes any longer than that, there is a risk of endometrial hyperplasia (the lining gets too thick) and it could become cancerous.
I have polycystic ovary syndrome (PCOS), a metabolic disorder in which insulin resistance causes my ovaries to produce too much testosterone. Consequently I was having trouble ovulating regularly. I was offered the options of the Pill (which I tried before marriage - women in our family don’t react well to the Pill and it gave me horrific mood swings), progesterone daily for a week every 3 months so that I would get a withdrawal bleed when I went off it, or do nothing. We had thought about trying for a pregnancy so my doctor started me on metformin, which I’ve been on for a few years now. Interestingly, it seemed to regulate out my cycles so that I do ovulate and menstruate on a fairly regular basis. While we have not been successful at conceiving (we think I had a miscarriage at 9 weeks at one point) I have stayed on it.
I’m not necessarily saying that metformin is what’s going to work for your friend as I don’t know her situation or her medical history. She’ll need to speak to her doctor. However, you might consider encouraging her to ask her doctor to look for the reasoning behind her lack of menses. Normally, the tests for this are fairly non-invasive - ultrasounds and blood tests to start with. Introducing her to NFP might also be helpful so that she can identify her body’s normal cycles; this can assist in determining whether or not she’s ovulating.