New Evidence! Help!

I have a big problem, and I cant find the answer on my own. Last year, my brother in law was murdered in a parking lot. The man who shot him claimed self defense (he said my bro hit him) and the prosecuting attorney didnt have enough evidence to prove murder. They offered a plea of manslaughter 1, and it was accepted. The man was sentenced to 13 years. Long story short, 1 year and 9 months after the original incident, I started a new job and was discussing the issue with some coworkers, when one girl piped up and said "Oh my god my friend saw that!" We discussed it some more and she contacted her friend, and discovered it was definitely my brother in laws case she had witnessed. The kicker is that what she remembers is NOT what we had all thought happened. In fact, it completely contradicts the perpetrators entire story. She never came forward initially due to fear and the fact that she was absolutely disturbed over what she saw. My question is, if she came forward and made a statement now, would it affect this guys conviction? Could he wind up with his charge and his sentence increased, or would we be able to get a new trial? He murdered my brother in law in cold blood. We just couldnt prove it. Nobody even knew there was somebody who saw it happen. Please help me figure this out, it means everything to me. Thanks

Have her tell the DA--it is hard to open closed cases but they may do so--or at least they may factor that in when it comes time for parole etc.