Another Reason Not to go to Law School

A friend asked me to post this legal question for a friend of his. He was afraid that if he posted it for his friend everyone would think it was for himself.

See this friend of my friend was driving down a hill without riding his brakes and blew through a yellow/reddening light at the intersection. A cop was sitting at the light on the crossing street (meaning 90 degrees across the intersection waiting for his light to turn green), saw him, and pulled him over. The cop accused him of speeding and running a red light, both of which could have been true. However, he had no evidence of the speeding (no radar reading) so he just wrote a ticket for not stopping at the red light.

The question is, was this cop wrong for writing the red light ticket? From where he was sitting at the intersection there is no way he could see what color the light in that direction was – he was facing the wrong side of the light. He could assume what the color was, but can he testify that the violation happened if he could not have witnessed the color of the light? Is it worth pleading not guilty and trying this as a defense?