Credit card insurance will only cover for car rental from commercial rental companies (Hertz, Avis, etc.).
I came across the following in a web article that might be helpful:
2. Have a Friend Add You
Perhaps you're not planning on renting a car at all, and you'll instead be borrowing a friend's or family member's vehicle while you're in Europe. In that case, you have a couple of options for insurance coverage, including a temporary add to the car owner's insurance plan.
This is probably the simplest option if you know your friend's car will be your primary mode of transportation. Ask the car's owner to add you as a driver to her car insurance policy.
3. Buy a Stand-Alone Policy
This option can help cover you in Europe, whether you plan on renting a car or borrowing one from a friend or family member. You can buy auto insurance for one specific time period, called trip insurance.
Trip insurance varies in cost, depending on the length of your trip, where you plan to go and how much you'll be spending. For example, trip insurance to cover a car rental in Italy for one week can range from $108 (with Assistance USA's silver policy) to $381 (for Travelex's Travel Max policy), according to insuremytrip.com, which provides comparisons of insurance policy prices.
If you're applying for coverage to drive a friend's car, you'll probably have to verify the identity of the vehicle's owner and/or registered keeper. Some insurance providers will only provide you with coverage to drive another person's car if that car belongs to your spouse, partner, civil partner, parent or employer, or a lease company.
Getting insurance to drive someone else's car can sometimes cost more, because insurers see you as a higher risk if you don't have financial interest in the vehicle you're using.
