Strictly speaking you do not have to be the registered owner as the registration documents are not proof of ownership. You MUST have an insurable interest in the car. Speak with your insurers and tell them the true position. If you are up front with them and they accept the position, then they will note their records that you are a type of bailee of the car and also note the owners name in the policy, that in the event of a claim being made against him, your policy will indemnify him as OWNER and also in the event of them making a payment in the event of a total loss, the cheque will be in joint names of you and him.