For starters show her some empathy and compassion!
She's clearly having a worse time than you did! Not everyone is the same.
Secondly use that empathy and compassion and some anger too to get her a referral to a gynaecologist. Which it sounds like she's never had?
So the gp has merely been "treating" the symptoms and not very well?
I was stuck on the pill - as so many are - which in most cases merely masks the problem until the patient then ttc at which point they then often go through the heartbreak of infertility/infant loss before finally getting a dx
I have endo which went undx for 14 years and lost 3 babies due to the woeful lack of support for girls and women suffering from gynae conditions.
It still takes a rough average of 10 years for sufferers of the main conditions to get a dx let alone find adequate treatment.
From her symptoms I suspect endo here too - but I could be wrong.
She NEEDS to see a gynaecologist! Endo rarely shows up on a scan unless very extensive and in the uterine area (it can be anywhere in the body I've even got some on my heart and lungs)
Wow! You've not even seen a dr about this? Why?! If she was suffering to this extent with any other bodily system I'm guessing you'd be pushing for better dx and treatment - remove your internalised misogyny and get her the help she needs!
I’m really surprised that they haven’t investigated PCOS or endometriosis I'm not! Sufferers are frequently fobbed off, get tranexamic acid, decent painkillers and put on pill IF they're relatively lucky!
Don't wait until September it can take months to get a specialist appointment and that's if you've a decent gp who's happy to refer (and they're generally very reluctant - it impacts them financially)
At least get the ball rolling before she starts back at school as that seems to be your main concern
Re hot water bottles - I found heat gave a little relief but that actually a cold filled hot water bottle (if that makes sense?) helped more as the pain was really caused by heat and inflammation but also NOT directly against the skin.