I hate to say this, but if you really don't think you can talk her out of it you need to either get her parents involved, or the police. I would never want to rat somebody out like that and I'm sure you don't either. But if you are dead serious and your friend BOUGHT A GUN and is planning on using it to kill somebody, she might possibly make the biggest mistake of her entire life.
I'm sure she must be feeling extremely upset and twisted up inside with anger over what happened to her friend, but she is "seeing red" right now as they say. If your friend is so blinded by rage that she actually goes through with this and KILLS the person who shot and killed her friend, she is going to throw away her whole entire life for nothing, and I guarantee she will regret it.
If her friend was still alive, she would want her to grieve but get over this tragic incident and move forward with her life. She would want your friend to grow up and be happy and get married and have kids and live her life the way that a normal person should, NOT wake up in a cell every single day knowing that her bad decision has her stuck in a concrete hellhole for the rest of her life. Unfortunately, murder is still murder, and vigilante justice will land her the same prison sentence that the murderer of her friend will surely get if she is not killed yet. Please, talk your friend out of this. This is extremely serious stuff and as her friend, it is your duty to do the right thing and stop her before she makes a complete mess of her life. She may hate you for it for awhile, but in the end she will thank you so much that she still has her freedom, and still has a life to live. DO NOT let her shoot anybody!