Trauma isn’t about the event per se. Numerous people experiencing or witnessing the same event may process it very differently.
Trauma could result from many different types of events, for example, birth, accident, childhood events, combat, bullying, DA, it’s about the response to the event. If someone experiences/witnesses an event or series of events so difficult for them or life threatening that they can’t process what happened, resulting in flashbacks (re-experiencing the event/s as if fully back in the moment), nightmares, depression, overwhelming emotions, then they might be diagnosed with PTSD (if enough time has elapsed for what they are experiencing to be considered outside the realms of an acute stress reaction to an event that has just happened).
With trauma, it isn’t just bad memories, it’s as if the brain gets stuck and certain situations can trigger a very visceral response. There are very specific evidence-based treatments such as EMDR and TF-CBT and often other approaches such as Compassion Focussed Therapy are used as well. There is a book called The Body Keeps the Score by Bessel van der Kolk which is an interesting read on the subject but not an easy read for someone struggling.
Ultimately we don’t know what’s happened to decide whether AA has PTSD and whether she should have or already has a formal diagnosis. But she doesn’t need to have been posted to Afghanistan before she qualifies for the diagnosis.