So treatment options are the pill or mirena coil - which I would really recommend. The mirena has a high chance of meaning she doesn't have her period, and that means the same hormonal changes don't happen. She likely also has MDD (major depressive disorder) or an anxiety with her PMDD, as it sounds like she likely has more than one mental health condition.
Has she had an assessment with a psychiatrist? Was she on the combined pill or the mini-pill? Also was it one she had a break for every 7 days or not? As there are different methods of the pill/dosage and just as one doesn't work for her doesn't mean another won't, as they are quite different.
However, if she doesn't want to go that route, then the other main treatment is antidepressants from after ovulation to when your period starts again. Something like propranolol could also help with anxiety. Anti-depressants are often life changing for this, and the benefit from taking them compared to not, is huge. ADs take up to 8 weeks to get working, so you need to give it time.
PMDD is caused by an abnormal brain response to normal hormone fluctuations, particularly in the luteal phase, when progesterone levels fall sharply. In PMDD, the brain’s sensitivity to these hormonal shifts is greater than in people without PMDD, which is why the pill or Mirena can help by stabilising hormones (the mirena often works better). The mirena gives more progesterone and also thins the uterus lining, and so that means there is not a lot left to bleed, so periods are very light or don't happen at all.
The progesterone also means that she'll have more progesterone, which is what is also missing that causes the PMDD.
It’s also why SSRIs are the most effective non-hormonal treatment, because serotonin levels and receptor sensitivity are biologically affected more than environmentally. SSRIs work directly on this serotonin dysregulation.
You can self-referral to local IAPT for therapy or or via your GP (search your local area and then IAPT self-refer and the link should come up). If symptoms are severe, which they sound like they are, ask your GP to refer her to psychiatry or perinatal mental health services as they are very equipped with PMDD. Perinatal services will treat you even if you don’t want to get pregnant, as many perinatal mental health teams also manage PMDD and hormonal mood disorders outside of pregnancy.
I've attached some links which should be useful on treatments you could have. The first is about getting a diagnosis. I hope she gets the support and treatment she needs.
https://www.mind.org.uk/information-support/types-of-mental-health-problems/premenstrual-dysphoric-disorder-pmdd/treatment-for-pmdd
www.mind.org.uk/information-support/types-of-mental-health-problems/premenstrual-dysphoric-disorder-pmdd/treatment-for-pmdd/