i try to remove myself physically from the situation. once i can do that, depending on where i am, i like to be in complete darkness, and/or shut my eyes and just rock back and forth while trying to remove all thoughts (good and bad) from my head...i'll usually just concentrate on the rocking until this happens. by then i've calmed down enough that i can function somewhat better. i haven't had many public meltdowns, luckily. happened a few times in school, a couple of times at work (not advisable, btw! lol) and once really badly this past xmas in front of my family (usually i have been good at hiding it from them in the past) that didn't go over too well, but made them aware of my issues, and maybe even a little more understanding.
but yeah, screaming and yelling (no matter where you are) does not tend to go over very well, i've found. good luck to your friend.
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friends are like balloons...once you let them go, you can't get them back.
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To the world you might be one person, but to one person you might be the world.