I'm sure there is more her GP could be doing. Perhaps suggest to her that she could seek a second opinion, from another GP, maybe one that has an interest in womens health. Or even see a gynae, if she can't find a GP who will help her. Also, natural therapies could be very helpful, if that's something she would consider trying. Acupuncture, or naturopathy.

You're a lovely friend, it's nice that you're so concerned for her
That's good advice. I had a lot of help with my cycles and hormones from natural herbs and supplements after seeing a naturopath. And getting a second opinion would be good - not menstruating for no apparent reason isn't normal and her GP should be helping to find out why. It might just be a hormone imbalance that's easily fixed.