In my opinion, the onus is on everyone playing the game. Not every person with PTSD knows what will trigger them. If a game goes long enough, someone will not

In my opinion, the onus is on everyone playing the game. Not every person with PTSD knows what will trigger them. If a game goes long enough, someone will not recognize they have stepped on a trigger they have been warned about. If everyone assumes good faith and treats accidents as accidents, there isn't really a need for anything dramatic. The DM's responsibility is to foster an environment where this can happen.