Either they're intentionally wearing a seizure-inducing costume, or they're not. It's impossible to be certain, but that's no reason to drop the topic altogether.
> If you have proof, or more info > Noone knows what set this person off
These are extremely odd questions to ask in this context, and imply significant insider knowledge about this community. Can you give us the context we're missing that helps us understand your background with flashing LED costumes?
> If that person was an asshole truly, they wouldn't have moved
Incorrect. Behaving politely and saying "oh, I'm so sorry" is the appropriate action to take, but offers no guidance whatsoever as to their intention.
Was their intention to trigger seizures in others, or were they oblivious to the problem when they designed the costume?
We don't have the evidence to say with certainty what the user's intentions were, and probably never will. But we still need to have a conversation about seizures and user intentions.
This is the human brain we're snow crashing, not just a program, and we owe it to ourselves to try harder to solve this problem. Intentions must be assumed sporadically malicious, as with all Internet-based things, and we should have — and have not — protected these folks from harm.
EDIT: Direct accusations about the individual aren't cool outside of their tight-knit community, because it's not our place to pass judgement or step in as moderators in their (VR-based) forum. It's still okay to discuss culpability of both parties in general and especially in VR and so forth. This is Hacker News. What can we hack together that can be used to filter seizures? Is there a photosensitive material that reacts to strobing by going dark until the strobing stops? Be the first to the finish line to make the danger-sensitive glasses from Hitchhiker's Guide!