You will need to notify the insurance company that had the policy on the rental car and explain the situation to see if the terms of your policy with them will allow them to accept or deny the claim.

There may be a basis for the claim to be denied since the person was not on the rental agreement as an additional driver on the vehicle and thus the insurance company did not give them permission to operate the vehicle. Also if this was an insurance policy that you bought at the rental agency it likely only covers you or others listed on the rental car contract.

Again without knowing the terms of your insurance policy placed on this rental car we are unable to give you a definite answer so read through the terms of your insurance policy and contact the insurance company that you have the policy through to see if they will accept a claim stemming from an accident your friend had while in the rented vehicle.