You should get her referred to a gynaecologist, and if possible specifically to one of the specialist centres accredited by the British Society for Gynaecological Endoscopy. This is a big red flag for endometriosis - not necessarily even for future endometriosis, but for actual disease now, and this is what the studies are showing. Many GPs and even some old-school gynaecologists are unaware of the big body of research that demonstrates the high prevalence of the disease in teenagers with these types of symptoms.
People who've had this but don't have a diagnosis, it would help if you don't keep saying 'I've had this and there's nothing wrong with me' unless you have had a laparoscopy performed by a specialist in endometriosis - other gynaecologists can and do get it wrong, and there is no non-invasive test to demonstrate its absence. Ultrasound is notoriously unreliable and wrong far more often than right, unless it's in the hands of one of the relatively few consultant radiologists who specialise in ultrasound on endometriosis patients.
Anyone who has this, it's worth getting it diagnosed because unfortunately if you turn out to be one of the severe cases, it can be extremely serious. If you're in your 40s, have had symptoms for a long time and are still undiagnosed, you don't know for sure that you're safe from some of the more extreme health problems it can sometimes cause with very little warning.
Sorry to sound scary but I do think it's worth being aware.