I am so sorry for your loss but my heart is really aching for your 14 yr old. I had a similar experience when I was 12 (I'm 28 now). My dog got killed and I felt like it was my fault; however, it was by a car and not by a pitbull. Unfortunately, I saw the whole thing happen and was really traumatized by it for a long time. I wish I could give some great advice on how to help her but I don't
Just give her lots of attention and do things to keep her mind off it if possible. Again, I'm so so sorry for your family's loss.