Oh dear, she is young to have such problems! (I don’t mean in any way that this is unnatural or abnormal, or means it can’t be MS - it can strike at any age).
It is common for symptoms to be quite variable. Many people find tiredness, stress and heat are all things that tend to exacerbate symptoms.
I am NOT a doctor, but it’s quite probable the chest pain is something colloquially known as “the MS hug”. MS tends to cause muscle spasms - could be any muscle - but when it’s the tiny muscles between the ribs (called intercostal muscles) it can cause chest pain and a feeling of tightness or constriction. At 17, it seems very unlikely it’s any sort of heart problem, and much more likely a muscular thing related to her presumed MS - nevertheless, chest pains should always be reported.
It is possible to treat symptoms, even in the absence of a diagnosis. IF it’s accepted the chest pain/breathing difficulties are likely a type of muscle spasm, muscle relaxants might help. Frequently prescribed examples are Baclofen and diazepam (I take both of these).
However, if she (I don’t like to keep saying “she”, but there is no special word for a son’s live-in gf) already suffers from muscular weakness, there is a risk that muscle relaxants may worsen this, so it’s all a bit trial-and-error, and finding the right balance. Some people get on great with muscle relaxants - no problem. I am one of those. But I did not have particularly severe weakness in the first place. Stiffness/cramp are among my worst problems, so anything that helps the muscles to relax tends to ease those symptoms.
I’m afraid diagnosis is often a long and drawn-out process - not because of foot-dragging, but because it’s a very difficult illness to diagnose, as there’s no yes-or-no test.
It can only be diagnosed by a specialist MS neurologist, and even they may need a long time and a lot of evidence before they’re certain enough to diagnose. Some people have just a single MS-like attack. Because MS is multiple by definition (it’s in the name), it usually cannot be diagnosed unless or until it happens again.
So I’m afraid there’s not much you can do to hasten things. A GP certainly can’t diagnose, so you are dependent on the neuro appointment, and even he (they’re mostly he) may not be allowed to diagnose on the evidence so far, however much he “thinks” it’s MS. There can be a big gap between what they think, and actually being able to prove it - which takes a long time.
All you could try is emphasising to the GP how bad she is, in the hope they can use a bit of influence to speed up the appointment, or perhaps grab a cancellation if one becomes available.
I’ve absolutely no idea of your circumstances, but I’ll mention it in case: you could consider seeing a neuro privately, which is usually much quicker (and often the same person you’d get on the NHS anyway), but usually of the order of a couple of hundred pounds.
As she has already had an MRI scan, you would not have the expense of that (as long as it can be made available to him), but if he orders any more, it would almost certainly be back to the NHS anyway, as they typically cost several hundred pounds, putting them beyond the reach of most ordinary people, unless they have health insurance. At 17, I’m assuming your son’s gf wouldn’t, but it’s just possible she is employed and has cover through work. I don’t think many 17-year-olds do, but I’m mentioning everything, just in case you realise: “Oh yes, she might have!”
I don’t think she would be covered under any sort of family health plan, because she’s technically not a relative.
I think perhaps you need an assessment for some practical help for someone who cannot shower or dress themselves (she is probably entitled to benefits in respect of this, as well).
You might want to talk to the CAB, or perhaps try the MS Helpline (number at top of page - though I don’t think they’re open weekends), for advice on who to contact, and what help (financial or other) might be available.
Hope at least some of his has helped,