Hi Ron,
I really can't answer what causes people's panic attacks, b/c i don't know. From what i hear, past events and traumatic experiences can definitely cause panic attacks. Sometimes, the person isn't even aware that this is the case. The hard part is figuring out what causes them.
As for me, there are no apparent triggers for panic attacks. Virtually anything can cause one. Thats part of the frustrating part, is there's no control. However, i can say that panic attacks don't happen to me if i'm in my comfort zone. Depending on how bad my anxiety has been at a certain time, my comfort zone gets bigger or smaller. Maybe it would help your friend if you were to learn what her comfort zone is, and why it is comfortable. Then you can be sensitive to this, and help create it, or maintain it.
I hope that helps. Bottom line is though, there's a good chance that she doesn't know what causes her panic attacks, although learning what makes them worse can help, b/c you can prevent those situations. Best of luck!