If they have bullets, casings or evidence that shots were fired in the car, the friend's testimony is going to be called further into question. ALSO the friend needs to make this looks like all the cops fault because if he is involved in a cop assault he can be held criminally liable for the resulting death of Brown.
Also, the autopsy can reveal if Brown's hands were up or down when he was shot. The cop knows this, I doubt Brown's friend knows this. This is why it is suspicious the family wants another autopsy..
Really? How can an autopsy do this? (seriously, I'm curious)