There are 3 likely options
Pilot incapacitation
Mechanical or software failure
Disorientation
Or a combination of these. Whilst it is not impossible that the fog played a part in this tragedy, it still remains to be proven.
The issue of the FAA license is at best a contributory factor and IF true that technically his license was invalid, even then it will not be the principal cause. I am saddened by the punctuation and formatting that some poster has decided to employ, as if he had found the best thing since sliced bread, as well as by his decision to initially not post a (relevantly) important bit of information (the Dutch medical). If he has been in conflict with the deceased (whom he obviously knew), so be it, but this is no place to give the final blow.
I did not know the pilot, the only common ground is that we are both Dutch and both love GA.
Yes the truth hurts (and will hurt the family, not the deceased), but the loss of their loved one will hurt more I am sure, so lets be as factual (-ly complete) as possible, devoid of emotions and keep each other up to date on developments whilst showing respect for a fellow GA pilot