The buddy scenario described by the OP is more than a little disappointing.
Somehow the buddyless diver has never realized that, if he doesn't know where his buddy is, then he has no idea where his emergency gas reserves are.
That's just pure ignorance. Sad.
One would think that his OW instructor explained that subtlety to him. Perhaps it was explained...but not understood. Perhaps, as offered by NWGratefulDiver, the diver was trying to juggle so many things (operating BCD, keeping breathing under control, kicking, following the instructor) that he ran out of mental bandwidth to attend to his buddy, the rest of the group, and the environment in general. Poor situational awareness. Not to excuse the behavior, but I could see this very easily happening to a beginner. I doubt the diver in question had much experience. Hopefully, all of this was discussed in the post-dive debrief.