My wife has it - she was diagnosed four years ago after a bout of pneumonia which hospitalised her.
Diagnosis tends to be the final option when everything else has been assessed and disgarded.
For almost a year she was virtually bed-ridden. her speech was slurred, her memory shot to hell, she ached too much to move except (with help) to the bathroom.
Now, with regular physio, she manages her condition very well - but still has 'wipe-out' days, sometimes two or three together - about once a week.
It is a misunderstood condition, because the major symptom is 'tiredness' - and we all get tired. The fact is, ME sufferers endure a level of exhaustion unknown to anyone else, and it should be more understood, and researched, and understood than it is.
My sympathy for your friend - tell her to eat as well as she can, and get some phsio via her GP when she is up to it.
A lot of sudderers manage to get a quality of life back, but you have to learn to manage it, and listen to your body's messages concerning rest.