It seems unlikely that Spock "witnessed it." Spock knew he was short on time, most likely he traveled directly from Vulcan to the supernova, why would he make a side trip to Romulas?
This would naturally follow if the supernova were Romulan homestar blowing up. And this is exactly what the movie shows happening.
I mean, there may be ambiguities in what the movie tells. But what it
shows is a continuous pan from the planet Romulus to a nearby star that then blows up. Just go watch the scene...
This is also the one and only way for the incident to make astronomical sense. A distant star blowing up
might spread FTL destruction by some unnatural Star Trek fashion, but what we see happen to Romulus is a very slow wave of destruction pulverizing the poor planet. And Nero swears
he saw the planet die, and immediately thereafter caught Spock in the vicinity of the timehole that Spock had created to swallow the supernova. Why would
Nero make a "side trip" there? It only makes sense if "there" is at Romulus.
We still don't know what Spock hoped to achieve exactly. When he deployed his red matter, the supernova had already burst and Romulus had already been pulverized, yet Spock at that point still claimed he "had little time". OTOH, what good would it do to turn Romulus' homestar into a timehole? So perhaps the plan all along was to let the star explode, and
then apply the red matter so that the star would revert to something benign. Hard to tell. Nevertheless, we have no reason to think Spock was outright lying about any part of his mind meld narration of the events to Kirk: he could well have been right next to Romulus when the planet died, or at least within the usual visual sensor range.
The part of the mind meld that included the destruction, was either Spock imagining the destruction, or perhaps a remote image he was sent.
Spock has this Alderaan thing down pat - he can tell when a shipload of Vulcans die, and he can "see" Vulcan itself die despite being stranded on a clouded-over little planet somewhere far away. But him seeing with his own eyes the pulverizing of planet Romulus up close is more likely than any alternative.
Timo Saloniemi