In the case of a 'rear end' accident, the one who hit the rear send is held completely responsible.
The injured car can file a claim directly with the offending caar's insurance. It will not jk on the injured car's insurance record.
The insurance company will get ahold of their insured driver to ask him his side of the story. Because in this case there is no police report or any other independent corroboration, if the driver disagrees with the injured cars story, the insurance may hint oay out.
Because of the time delay in going the insurance route and because it sounds like a minor expense , if your friend still has he car, he should probably just eat the cost of the repair. .