Sure sounds like something in the steering column. Is his left turn signal on when this happens, or happens every time the wheel is turned left with the signals off?

I did have a '78 Magnum, where the interior lights would come on when you pressed the brake pedal (and the brake lights did not work). Turned out to be a bad fuse in the brake light circuit, happened more than once. Replaced the fuse and brake lights worked normally.

Could be the door switch also if the door is loose/ wiggling. Has he had the doors off recently?