It sounds silly but it happened so quickly he was doing about a ton.
National speed limit road, he overtook on double white lines in a dip with a blind summit about 50 meters away with a right turn which i fortunately wasn't turning into.
Funny thing was i caught up with him about 5 miles later i was doing the speed limit and no more, as he was about to overtake 2 cars in one go with oncoming traffic who flashed him. He then got caught at some red lights so i informed him the error of his ways, which fell on deaf ears of course.
I haven't actually done anything about it.