"The part of the picture you're missing is that he was off duty."
=========
Perhaps officially. But cops are cops 24/7....carry a gun off-duty...and are normally authorized and even expected to take action if the "need" arises in front of them....like responding to a woman screaming for help or a person who needs medical attention, CPR.
-
Seeing someone "speeding" is not normally something any department would encourage an "off-duty" officer to take action on. Hit and run? Maybe. Speeding? No. 
-
But he did identify himself and plainclothes officers, detectives, and investigators contact people all the time for various reasons....including investigating crimes and making arrests. 
-
So she (1) should have known he was a cop (2) didn't know he was off-duty. 
-
Regardless, his department will review his actions....and apparently the court will review hers.