The best way to help someone "panicking"?

I don't even know if "panic" is the correct term for this situation but nothing better comes to my mind. The other day I was with a friend and she saw something bad regarding her, actually not really serious but it hit her near home. At first she was just a bit sad but after 15/20 minutes (I think this was the time to elaborate the fact in her mind) she started to shake really fast her leg without being able to control it and couldn't stop crying. I managed to calm and help her but I had to figure out how to do it in that moment and still don't know if it was the most efficace way. So, what's the best way to calm someone in a similar situation? Hoping this is the right subreddit, thank you for your answers