Clifftop, I'm really sorry to hear this as it can be a nasty thing. I had it years ago at the same sort of age. I would say that you will all have to accept that this type of post-viral thing goes at its own pace. Please don't encourage your granddaughter to do more than she can manage. Always allow her to rest if she is tired. She may recover quite slowly and again, if this is so, she mustn't be overtaxed or made to feel that she isn't getting better quick enough. This is very important at the beginning and she will probably be quite depressed and upset about it, so do try to reassure her.
I'm sure you and her parents have encouraged healthy eating, as much as she is able. If she appears not to be recovering normally from this, you could ask your doctor for a referral to a specialist. My CFS resulted from Mononucleosis and took me 2 years to get over fully with occasional lapses for a few years afterwards. A friend had it at the same time and was well within a month. I have since encountered it in others and all have had different recovery times.
If her doctor isn't sympathetic try to get another doctor onside as it can be difficult enough without the medical profession telling you it's all in your mind. Any help I can be please let me know. I am not a medical professional by the way, and wouldn't want my advice to supersede proper medical advice. But always get a second medical opinion if you aren't happy. Best wishes, Trish