How do you calm down someone else during a panic attack?
It really depends on the person
Some people would react well to a hug, or some kind of physical contact, but others won't. Ask before. Make sure that the person keeps breathing at a steady rhythm, so they don't hyperventilate.You can do this by saying " Okay, breathe with me" and do really slow, loud, deep breaths while looking at the person in the eye.
If they let you hug them, try to be calmed, and your heartbeat can calm them down too. Keep talking and saying positive things, reassure that you're there with them, that you're not leaving. Don't make them speak unless they want to talk, don't force them to do anything, just be there as they need you.