Okay. I'm looking at People v. Stamp: "as long as the homicide is the direct causal result of the robbery the felony-murder rules applies... ." So if you can show that but for the D's armed robbery, the customer would not have died AND that people running amok is a proximate and forseeable consequence of carrying a gun into a store, then under that reasoning, I think it's still felony murder.
However, I'm also looking at State v. Canola which refused to extend liability beyond acts by the felon and his accomplices, to lethal acts of third persons not in furtherance of the felony. So under that ruling, you could make a strong argument that he can't be held liable since the killing was at the hands (feet) of a third party. I think that used to be the majority rule, but maybe not anymore.