The person who hit your brother is 100% at fault. Ironically enough, of all people my fucking mom pulled that same move about 25 years ago to a mail truck. Her fault.

If the police report says that one guy is at fault and that's it, they can't go back in time and issue a no signal ticket. Simple as that. Your brother can basically call up the other persons insurance company and tell them to pay up, shut up, and fuck off. Don't admit anything to the other insurance company. It's not required.