Of course, B12 should be tested for. Many alarming things happen when
B12 is very low. They often look like MS. You don't say how old this friend is.

There are extreme neuro diseases that can come up this way... a fast
presentation...one is nvCJD disease. But this is very rare in USA.
Where does he work? Could he have been exposed to some toxin at work?
Solvent, or heavy metal poison?

But another avenue that should be looked at is heavy metal poisoning.
A blood test should be done for this too. It can be a very sneaky
type of presentation.
http://www.lef.org/protocols/prtcl-156.shtml

Has he been ill? West Nile Virus causes an encephalopathy in some people.
Meningitis causes a severe headache in the beginning.
So infectious things should be ruled out. Did he have a spinal tap?