Pepper Plants wilting

Question from a close friend, who grows a lot of peppers (bell + 4-6 hot varieties).  He has experienced following (both last year & this year):

A mature plant suddenly wilts the youngest leaves on the top.  After a few days, the rest of the plant wilts.  The roots look healthy, but the plant is dead.

It happened to 4 plants last year (in 2 different raised beds), and has happened to 2 plants so far this year.

Any thoughts on what is causing this, and whether there might be a treatment?