G.♣.
If the guest hit your brother's car, I would think the guest would be at fault. If that's the case, the guest's insurance should pay for it. (And aren't most people required by law to have liability insurance?) If the guest has no insurance, I think they are still at fault and are supposed to pay, but if that isn't a possibility, your brother's regular car insurance can pay.
I've never taken out the additional insurance, because the insurance I carry on my own vehicle will cover me (if I were at fault) if anything were to happy while driving a rental car.
If he hasn't already, I would take pictures and call the local police to make a report. The insurance companies will all want to have copies of that report as they try to figure out whose at fault and who will pay.