Hi Katherine, I can understand your frustrations, this illness is so debilitating and there is little help nor good advice from the NHS, who don't seem to take it very seriously.
I found optimising my diet can really help improve energy levels and using their pacing protocols have been invaluable. Diet and lifestyle changes take a lot of work and energy and can take some time to show results, so if you can help with diet and pacing it would be really helpful for your daughter. Also making sure when it gets bad, she pulls back a bit and concentrates on healing things and not put so much pressure on herself to push through it to 'achieve', but to focus on expanding wellbeing and finding more long term plans to reach her goals. When I lost my successful dream career in the film industry to this illness I was bereft, but I have mananged to create a website and hope to create youtube videos and write screenplays and I am slowly working towards these things in the long term. This illness is devestating. I underestimated it in the beginning as my original GP regulalry ridiculed me, so I didn't appreciate the severity of my own illness and just kept throwing myself back into work as soon as I could. As a result my illness got steadily worse I had to fight to get CBT & GET on the NHS and it pushed me into a relapse so severe that I could no longer work. I wish I had known then what I know now and to focussed on recovery before taking too much on before I was ready. It's a long haul to recovery and it can seem precarious and can take years, but I am inspired by the recovery stories on the freedom from ME site and it gives me hope of a brighter future. Until someone comes up with a cure, the best thing we can do for ourselves is to focus on expanding our wellbeing and try to give our bodies the best tools to heal itself.
Your love and support is clear from your message and I can gurantee that is making a huge difference to her wellbeing. 
Yes following diet and lifestyle changes can seem really restrictive at first, but it has really helped me to get out of severe ME and into moderate and hopefully continue to mild and maybe even full recovery!! :D
Useful Stuff:
Low GI organic diet with protein for every meal, AND don't skip meals! Ever! It's really bad for the adrenals, which are already under stress from being ill.
Avoid caffiene, stimulants, sugar, bread, white refined carbs, ready meals - This is really difficult as we tent to crave sugar and caffiene when we're energy deplete, but these things are really bad for someone trying to recover from ME/CFS.
Supplements: It's best to see a professional like the OHC to find out deficiencies and use high quality supplements. Make sure deficiencies are tested/ruled out by blood tests D, Iron Magnesium etc with your GP Also make sure other illness are ruled out with your GP such as thyroid, diabetes, etc. Magnesium supplementation made a huge difference to some of my more severe symptoms. Look for "Magnesium – the Forgotten Mineral - Nutri" for info. Don't overdo it though.
Build gut health with probiotics - lots of problems are caused by not having enough good bacteria which boost immune system. Search Leaky Gut syndrome and see if this could be a contributor. Some of the lastest ongoing ME research is a gut microbiome study. Also a recent study on allerges shows they may be caused by lack of certain good bacteria in the stomach and many people with ME/CFS find they become intolerant to foods and have difficulty tolerating supplements. Many also have vitamin & mineral deficiencies which may be caused by leaky gut, so even if you have a healthy diet, you're not getting the nutrition you need.
Omega 3 oils: There was a study showing abmormal brain function in people with ME which was helped by EPA oils, you can read about it in the book "Chronic Fatigue Syndrome: A Natural Way to Treat M.E. by Basant K. Puri" I can't tolerate fish oils right now so I am using Epigenics Red Wonder oil, which is plant based.
Take up Tai Chi or Yoga. I do yoga stretches every morning and night, but don't get on with full yoga (which makes me dizzy) so do Smile Qigong when I am able. Smile Qigong is tai chi designed especially for people with ME/CFS. Gentle walking is also good.
Pacing: Pacing is really helpful and pacing can be different depending on illness stage and severity. I find the Freedom From ME pacing protocols the best I have used and can also recommend "Tired of Being Tired' book by Dr Jesse Hanley is a great book for pacing and diet for people with fatigue.
Also search for 'Natural remedies for POTS' if your daughter is spending a lot of time immobile and is now finding dizzy on standing and can't stand for long. It's good to try and prevent this as many ewho spend too much time lying down may be sucepable.
Amygdala Training: The guy who desigend this programe believes that illness is prolonged due to damage to the amygdala (possibly by inflammation in the brain, which could be casued by virus or something like leaky gut or Heavy metal poisoning (like from mercury in Tuna, etc) and he aims to retrain the amygdala so the body can heal without sending all the faulty signals from the brain. I've only just got this so can't say if it works, but I think it's better than similar therapies such as Ligtening Process, Reverse Process, Mikel Therapy as you can do it on a 5 dvd course from home, it's cheaper at around £100 and Dr Gupta offers a money back guarantee if it doesnt work like you hoped in 6 months. I think he's the only person offering a money back guarantee for this type of therapy. I've only done the first dvd so far and I do feel more positive.
Something that has also helped me, is rather than concentrate on what I can't do anymore, is to focus on what I can do and I feel it's very important to find things that bring you joy and make sure to include them in everyday life - some new hobbies perhaps that are fatigue friendly. I took up meditation and mindfulness. It helps me to focus on on the moment and value the little things.
Jon Kabat Zinn who teaches mindfulness does a CD on using mindfulness for pain relief and also wrote a very cool book called "Full Catastrophe Living: How to cope with stress, pain and illness using mindfulness meditation" and the CD is called "Mindfulness Meditation for Pain Relief: Guided Practices for Reclaiming Your Body and Your Life" I have found both really useful in coping with the challenges of ilness. I also found Chinese Traditional Accupuncture helful for water retention, headaches, pain and nausia and osteopath James Ellis for back pain and oedema along my spine. He used a similar technique to the Perrin one.
Don't lose hope. Together you will get through this. Hopefully some of the things that have helped me will be of use to you and your daughter.
I make my own oat crunchies and put only 30g of sugar in for a treat. I can post the recepie here if it helps. The OHC said to me that to aim for about 70% sticking to the diet as anymore can be too tough and swap little things - like I used to love scrambled eggs on toast and now I have scrambled eggs and asparagus. I find dairy too difficult to give up, so I switched to goat cheese and almond milk. I swapped crisps for roasted nuts and seeds, and try to use virgin coconut oil instead of butter. For lunch I have mainly salads with chicken or goats cheese or organic vegetable or chicken broth type soups - soups are great as they can be frozen. My mum makes me soups and separates in to servings and freezes them as it means something quick and hassel free and nutricious is always available and we know it's all fresh with not additives. I find soups the best when I'm feeling nauseus and drinking warm or hot filtered water. I can't tolerate cold water so I drink it hot. Ginger tea may help nausea. I also have aloe vera juice when my tummy is weird and gripey and sometimes in the morings I have hot water with a squirt of freshly squeezed organic lemon juice. Lemon goes alkaline in the body so it's very healing and a decongestant with lots of vitamin C. I avoid orange juice as it can cause migranes, asthma and in me personally cistitis and stomach cramps and stays acididc in the body, unlike lemon juice which turns alkaline when it's broken down in the body. Weird huh?