CliffordK

If you are not a witness, then the police won't necessarily want to talk to you, unless you are a "professional" with respect to first responders in some way. Doctor? Accident Analyst? Professional Accident Chaser?

I'd probably give them space if I didn't actually know the person.

If I actually knew the person, I'd offer to help facilitate collecting belongings. I'm not sure if the police would wish to keep the bicycle as "evidence", and for insurance to inspect. But, there may be other personal belongings you could track for a friend.

Pick up pieces left on the road? Lights?

Also, are the police trained to recognize items such as Fly6 and Fly12 devices? Other cameras or evidence? Strava and data trackers (which could be both good and bad for the cyclist, but important for any legal proceedings).