Light pollution is minimal. Burning objects are quite visible at great distances during the night. I spent 10 years in the area back in the 60s & 70s & its dark at night there.
The coordinates on the letter put the rig’s postion about 100 or so miles off the coast, to the north east of the last recorded position on your map. But he spotted it at around 270° from his position and having no lateral motion, which is odd. So the plane would have been going east or west, not north east.
He said he is on an oil rig. His coords put him well out off the SE coast of VN.