Moridin
Please check with your police department. Many of them have Crisis Units that will provide trauma counseling for victims of crime. It is usually free. Go... and give this information to this girl and the rest of your staff.
Expect that for a while you will be having a hard time emotionally and maybe even physically. This is normal. Trauma (and this was a traumatic experience) affects people in different ways. Talking about it helps - a lot. By talking about it you are able to process the experience and try to fit in into some kind of world view although I think most of us have a hard time doing that when it comes to an attack like this.
You may have nightmares and flashbacks - seeing it happen all over again. This too is normal and will pass in time. The more you talk and process it the easier it will be to deal with. Bottling it up often results in Post Traumatic Stress Disorder. If you can't find anyone to talk to then write it down - even talking out loud to yourself is better than bottling it up. Get all the support you can. You might want to consider all of your staff sitting down and talking about this together. Going through a trauma with others and then talking about it can help all of you collectively and individually. Don't push them but certainly encourage them.
Take care of yourself physically as well/ Make sure you are eating and sleeping even if the dreams start. Try doing relaxation exercises or visualizations to help you sleep. And if you need medications for a few weeks thendon't be afraid to use it. Trauma can put a load on us physically and emtionally.
Feel free to contact me if you need some more information