There are many things I might have forgotten but here is what came to my mind. To sum up, if someone is having a panic attack :

-find a calm place
-be there
-don't touch, hug, switch the lights off, etc, without asking and waiting for an answer
-take care for yourself
-it's scary but it has an end
-listen and pay attention
-do things slowly and remember the person might not have as much dexterity as usual
-avoid harsh lights, sounds, etc
-stay at calm after

Thanks a lot if you are here for anyone suffering from panic attack. Yet, take care for yourself too. Don't blame yourself for being the cause of panic or not knowing what to do.