In terms of OTC, my general idea would be to escalate from: Paracetamol > Paracetamol + Ibuprofen > Naproxen. It depends on if she's tried any of these so far, but when I used to get very bad period pains the Naproxen were the only ones that helped (at the time I bought Feminax).
I have relatives that get them so severely that no OTC painkillers will help; they take the likes of cocodamol or naproxen to take the edge off, but have been prescribed other medications such as the pill to help regulate and reduce the periods. They are also being examined for other possible problems.
This latter point is most important IMO. If they've very bad, then there could be an underlying problem that can't be diagnosed just by looking at her - she may need an ultrasound, for example, I believe this is part of how they diagnose endometriosis (never suffered myself but had a friend who did, she also had a biopsy done for ovarian cysts).
Frankly, she needs to go to a different doctor if he's not checking her over or treating the problem. If the issue is a language barrier then would you be able to help her arrange for an interpreter (unless you also speak the same language so could also help)? Perhaps someone better informed will come along with NHS-specific info, but there are various charities out there that will provide interpreters free of charge for appointments.
Heat pads and hot water bottles in the meantime can also help with the muscle pains/contractions, I used these in conjunction with naproxen as they would cause me terrible back pain.