My opinion is that in most cases the diagnosis is not helpful. The trouble with the term Chronic Fatigue Syndrome, in my opinion, is that it is not specific enough. There are probably many causes of the same complex of symptoms. The important thing is to figure out what causes his symptoms: adrenal insufficiency, food allergies, poor digestion, an unresolved virus, etc. In my case, this exploration took years, but I now understand that my fatigue and other symptoms are caused by low functioning adrenals glands. Knowing this, I can make lifestyle changes and take supplements to help. If your fiancé is diagnosed with CFS, he will still need to find out what caused it if he's going to make any real recovery.
This is only my opinion based on my own experience. I found that most doctors were not helpful, so I was not interested in their diagnoses. If you find a doctor who is experienced working with CFS, then the diagnosis may open the doors to recovery. But you need to be very careful in finding that doctor, because many doctors will not take CFS seriously, and others may advocate unhelpful and expensive therapies.