Whoa! That was probably a big surprise. My opinion doesn't have much that YOU can do.
This girl is not your daughter and what you saw NEEDS to be reported to her parents or guardians. If you can't contact the person(s) responsible for her (and for her behavior), you might resort to calling the Child Protection Services. I hope you reach her parents.
I have three boys (14, 9 and 6 years old) and found it important to teach them that there is a limit to teasing and scaring someone. Evidently your son thinks it's funny.
Here, you can't make this girl calm down or make her make better choices when she gets angry. You need to protect your son.
One of the lessons I have been practicing with my boys is that it matters how the recipient feels. If she is constantly saying "No" and "Stop" and "Enough", then she will feel unheard and will escalate her response, like in this case.
My boys found this lesson a hard one to grasp. When one of mine doesn't heed the "Stop" message, he gets in trouble. Pick a consequence if he is found trying to "scare" this girl. My first choice would be a quiet half-hour timeout in my room (away from toys, etc). Some loss of video game, computer or TV watching privileges could also be a choice.
Hope this girl gets help!