Man, that will be a tough one. Technically, you aren't supposed to "enter" an intersection (as he did from what you said) if the light is yellow. Yellow only means to CLEAR the intersection, not enter it. However, he could argue that stopping suddenly might have caused a wreck behind him as the light changed just as he was approaching the intersection. I'm afraid that there is probably nothing that you can do given the "devil's advocate" view that I am offering.