Calling 911 in reality has always been 50/50 for me.As a woman driving alone, I would not stop and engage with a man who looked like he had mud and blood all him.
I would call 911 if I thought he needed help. But I wouldn't speak to him unless it was obvious he was injured, which this guy apparently was not.
I have called 911 two times that I remember of: A man was trying to force a half-naked girl to the ground (turns out he was restraining her because she wanted to run into the traffic, that was a terrifying experience). A motorcycle crashed in front of me.
Times I should have called 911 but did not, because I was terrified: Multiple gunshots directly next to my house (I was scared that if the police showed up the perps would know which house had called). Going for a run and seeing two groups of men fighting with machetes (I turned and run the other way around, and didn't call because one of them saw me and was pointing at me). I did not call 911 in cases when a stranger pulled a knife in front of me, because it didn't even occur me to do so. Seeing a bloody male, probably seeing someone hit them, but I wasn't sure. (I didn't call it in because there were other people around, and it looked like someone had already called 911)
All this to say that in my experience, my expectation of what I would do most of the times does not coincide with what I actually end up doing... I don't blame the witness in the least, I honestly don't know what I'd have done in her shoes.
(Alternatively I am a terrible citizen, which is also a possibility.)
All MOO