It's not "judge-jury-executioner" and it's not "vigilantism" when the panic is still on. Never has been. Both those terms imply that (1) there is no continuing threat and (2) there has been time for reflection (to gather a lynch mob or whatever). If the victim had gone home, gotten her friend, and they both got in the Lexus and went out looking for this armed robber, that would be vigilantism.
This is well within what the courts generally call the "res gestae" - i.e. no time for reflection - and I have seen plenty of cases where the courts have held that an armed robber who is still ambulatory and able to reach for his gun is still considered a threat. Most self defense statutes also consider the stress that the person is under at the time . . . given that the driver saw her friend robbed at gunpoint, intervened to stop the threat, and the guy kept getting up and eventually pulled the gun again, it clearly would go to the jury on self defense. The fact that she's a woman makes an acquittal or even a DV far more likely. She will testify that she was in fear. It's up to the jury to decide whether that fear was reasonable, but given that this appears to have all happened quickly and continuously, it might be DV material.
Drugs and speeding are no comparison with a gun pointed in your face or in your friend's face. And car related deaths are overwhelmingly accidental, so that's a red herring too.
Sorry. I don't buy it. If the events happened as written, it was vigilantism in my book.