If you haven’t spoken with the person experiencing the flashback on how they prefer to be assisted, it is best to err on the side of caution. Trying to help, no matter the intent, can often cause more harm than good. Stay calm, and don’t ask too many questions. They are probably in too much of a crisis to answer them. Instead, step aside and give them space, but stay nearby. Let them know you’re there if they need anything, but then start using your phone or something so they don’t feel like they’re being watched. Be there if they choose to reach out, but leave the person alone if they are in no immediate danger.
Please, DON’T touch the person experiencing the flashback unless they have made it clear that’s okay. A hug or a hand on a shoulder may seem like it would be a comforting gesture, but the person is in sensory overload, and it may make things much worse.