I doubt anyone has seen this....perhapse because if you were close enough to see it happen you would never survive to tell the tale, I am not an astronomer but you would probably only live long enough to see a flash of light and nothing else, that is if you wanted to be close enough to see it happen in any detail. Would be great to see it though. I would sacrifice myself and the cost of replacing my ship in the name of science
Apparently the supermova that formed the Crab nebulae was visible for 23 days on earth way back in 1054. That happened 6500 light years away. So to see one and live to tell the tale or rather experience the aftermath of the explosion which lasts around 100 seconds, you would have to be so far away you would see nothing more spectacular than a star becoming brighter for a few days, say in a neighbouring system, which are typically several light years away from each other. So if one was happening now and you knew and were in the next system waiting to see it, you would have to wait several years in the game ( and real life!) for the light from the explosion to reach you. But I dont think it would happen even then as I dont think the simulated galaxy in Elite goes that deep as to simulate the time light is taking to travel between star systems.
But saying that Elite Dangerous plays fast and loose with laws of physics when it wants to. How can a little Sidewinder ship with no shields get close enough to a massive star to scoop fuel and suffer only minor heat damage, yet a laser or cannon firing tiny shells rip you to shreds in seconds?
LOL - i just turned a brilliant "what if..?" post and turned it in to rather boring science ..... I will shut up now