Is it possible that she's picked up a virus?? hence why she has suddenly deteriorated, once diagnosed its easy to pin everything on CFS, but you still do get other illness, often more so as it seems to affect immunity, so it could be a flu virus that will run its course if she rests properly & treats the symptoms. Or maybe as in my case now, an actual infection - I made the mistake of blaming it on the usual, but now realise I need antibiotics for a sinus infection - I find infections up my pain & exhaustion levels a lot.
As she's 15 if you organise properly can she not look after herself for those 2 days? Making sure she has plenty of drinks & snacks by her bed, is there neighbours who could pop in etc. - she might even prefer the peace & calm of being alone & not having to talk etc
There's quite a bit that can be done to help too, Epsom salts gets magnesium into your system & helps the body cope with stress & pain more efficiently & sleep better too, 2 cups in a hot bath & soak for 20 min or so, if she can't handle a bath, a foot bath will do - aromatherapy helps - add lavender & chamomile to bath or even pillow to help relaxation & sleep, clary sage can help ease pain & lift her mood, though this is not an oil for regular, or over use as it can have side effects, it is still well worth trying if things are bad - taking Spirulina is great for helping energy & immunity, it contains a LOT of good stuff, more iron than liver for example, but easily digested & B12 & much more - I can't take it anymore due to high pottassium, but I really do notice the difference - it got my DD back to School when she couldn't cope too. Also if she can cope with getting out at all, I found that sauna, or manual lymphatic drainage massage can help a lot - you might even be able to find a visiting therapist
I'm being a bit vague over my our diagnosis, I was originally diagnosed with Fibromyalgia & CFS, DD was thought to have Juvenile CFS too, but a few years down the line my DD has just been diagnosed with Joint Hypermobility Syndrome, & they now think that's my true diagnosis too - symptoms are the same as FM, CfS etc but treatment is a bit different, though pacing etc us the same.
So I've been dealing with CFS for 10 years, learnt that the trick above help us, though now finding its actually a different diagnosis, IYSWIM