He was calculating the cost of losing his job and having his house looted if he intervened...
“There was at least one witness to the attack, a man on the sidewalk behind the woman. He appears to freeze in disbelief as the woman collapses, the video shows.”
No calculations necessary.
The witness was walking behind the attacker and looking at his cellphone. Basically, the typical sidewalk zombie who, in this case, didn’t notice anything until after the attack was already over.