Whose Auto Insurance Applies When Driving Someone Else's Car?

Typically, insurance follows the car, not the driver.  That is why it is important that when you are driving someone else’s car, you inquire as to whether or not they have insurance on the vehicle.  And don’t stop there, find out what coverages they have—just liability, or do they also have collision, comprehensive, medical payments, uninsured/underinsured motorist?  This way, if you have an accident, you know what is available to you as well as to the other driver if it is your fault.  So the answer to your question is, if you both have insurance, your friend’s insurance company would handle the claim.

If you drive someone else’s vehicle and they do not have any insurance (but you do) and you cause an accident, then your own insurance will step in with Medical Payments (Med Pay) coverage (if you carry it) to handle your medical bills and liability coverage to take care of the damage you cause to another vehicle and their medical bills and related expenses if they are injured.  It usually will not, however, take care of the damage to the uninsured car you were driving.  (Your friend needed to have collision coverage for that.)