The Driving Other Cars extension provides the policyholder only with the minimum cover required by law to drive another car. So the damage to the car you are borrowing would not be covered- the intention of this extension is to provide the policyholder with cover should they need to drive someone elses car in an emergency. Be Careful as some insurers qualify this within the policy and state that this extension will not apply to certain vehicles e.g. cars belonging to other family members