"Post 2548 makes an excellent point... at 35000 feet, the distance to the horizon is ~229 miles, or 368 km."
But the deck of an oil rig is well above the sea so you would have to add the sight distance from the rig to the horizon.
Add in refraction then it might well be possible to see at that distance.
And if currents took the debris away from the rig it could well have been closer than where it was found.
Not saying he did see it (could have been a meteorite) but that it does not appear to be completely impossible.