I'm so sorry this happened to your partner and glad that he's ok. I don't understand. Even if you were off the route of the ride whatever happened to "bikes are traffic"? Day, night, whatever you have a right to be on the road. How else would you get to or from an event? What's wrong with this guy? (cop).

What everyone else said: gather witnesses, cell phone video or pictures, lawyer, local community police ombudsmen, cycling advocacy group ....