My eldest son would have total melt-downs like that. He really would beside himself with rage and lash out at everybody and anything, including himself, banging his forehead until it was bruised. All this was nearly fifty years ago. We now know that he was( is) on the autistic range.
We tried putting him into a safe time out place whilst these tempers subsided. He would smash anything he could in there.
All I can say, is that as he is got older, he learned how to manage these tempers, and will even now all these many years later still walk away from any sort of confrontation. He walked out of his 40th birthday celebrations in a restaurant, because the waiters came out singing Happy Birthday. He has an excellent career and a lovely wife and home.
My advice is to discuss this with the parents, they do need to know. Perhaps it is happening in other places. They may need to seek some sort of professional advice. Poor child, she is just totally losing control, probably feels really bad afterwards. Talk to her about it also, and any future one try to control by letter he get it out of her system, and she will probably ends up in normal tears and exhausted. At that young age, it is usually able to be controlled by wrapping arms around her tight, or else having a very very safe time out space for her to go.
I can remember when he was about seven years old, my Brother in law, who a strict disiplinarian, decided to intervene when he threw one of these tantrums at their new house.. Put him outside to 'calm down'!!! It was fortunate that I caught him just in time as he picked up a garden chair and was about to throw it at the large plate glass kitchen patio door!!!