I have a friend who was held up at gun-point a few months ago, and has nightmare!

I have a friend who was held up at gun-point a few months ago, and has nightmares.What can she do to get help?

Help her find a good counselor with experience in treating PTSD - Post Traumatic Stress Disorder. They used to call this "shell shock" but it can be caused by many types of trauma.
I have found that telling yourself - I'm here - I'm ok - I'm not in danger - helps difuse the most panicky and anxiety producing parts. If she has nightmares and trouble sleeping have her try getting in plenty of exercise - it will help both make her feel better by controlling and releasing the anxious energy and by tiring herself out so she can sleep without getting the nightmares.
Mostly as she feels safer it will disipate in time but can return when triggers happen - loud noises, people argueing, perhaps the sight of certain things - violent movies - can trigger anxiety.
Mostly knowing that it is normal will help difuse it.
PTSD is sort of a gift - it is natures way of gearing you up so you are ready for danger and ready to survive it.
If she thinks of it as a survival mechanism it may help her feel like less of a victim.
Self defense training or a group where she can talk to other women can be helpful too.
I definitely would NOT recommend any drugs for it - the symptoms will subside over time and most drugs to treat mental health problems have really nasty side effects.