Razoray;
So it is possible that the Captain set the course and left the cockpit for his break. And that the 1st officer remained on that course, not deviating to avoid the cells.
Of course it is possible.
However, having done this many times before retiring and knowing, with variations on the theme because the problems we are confronting are the same no matter the airline or airplane, that these things have much in common, the captain in his or her changeover briefing would not "set the course" and leave the cockpit expecting blind obedience. Nor does such a notion make sense.
If circumstances were such that this kind of directive were required, the captain would, and should, remain in the cockpit until the anticipated threats were handled and passed.
So while it is possible, it is almost certainly not the case. The flight deck crew must remain free to make operational decisions "in the moment" when/if circumstances require. Part of the briefing would include the requirement to call the captain to the flight deck if any operational matter out of the ordinary which, for example, required a significant change to the flight plan or required an emergency drill.
If the captain is uncomfortable leaving the crew for whatever reasons, he or she remains on the flight deck and assigns breaks as necessary, until such time as conditions permit a break for the captain. Such a situation would be very rare and as such is always complex and difficult to define. It could involve someone not feeling well for example; it happens and crews deal with these matters as they arise.
If you need a metric, I would not leave the flight deck in the hands of a First Officer who was brand new on the airplane, who had never been overseas before, and who had never dealt with complex weather systems in the ITCZ or demonstrated to me his/her ability to read and use radar information to navigate same, while in the company of a new, MCP-licensed Relief Pilot. Others will define their personal judgement of comfort differently. I have never had to alter rest schedules for such reasons.
PJ2