Well, its not as if my doctors didn't try to diagnose it. I got a virus panel for a lot of viruses whose names I can't remember. And, it was one of these doctors, young at the time, who showed me the CDC definition of CFS based on "6 of 12" symptoms. He said "we don't know what you have" but some people have been lumped under this syndrome. "Maybe some more symptoms will come and whatever it is will declare itself."
For me, I can't trace a specific causative agent, but I can trace a specific onset date. One summer day, when I was in my early 40's, I woke up with the most debilitating "out of it" fatigue, but no fever or other symptoms. It was diagnosed as "a summuer virus" but it lingered.... and lingered.
It took a long time ... maybe seven years... to fully abate. Of course, after all that time, who can say if it fully abated, because I was getting a bit older anyway. But it did abate, and my case wasn't as bad as some. I was usually pretty good for several hours a day, and sometimes for weeks, with frequent relapses. Acupuncture was somewhat helpful, but not totally. Very strange.
Good luck to your friend.
Sooner or later, modern medicine will figure out some of the causes. I'd bet on an extended immune reaction which persists after the initial threat has subsided. That's a common hypothesis.
To you and those whose posts echo yours it might be wise to consider that not every day can be a great day and that going to a doctor is not always the best first course.