You and your friend could both be responsible. You should each report this loss to your respective insurance companies unless your friend purchased insurance from the rental agency. Then, it should be reported to them. If your friend did not purchase insurance from the rental agency then a claims adjuster from each of your insurance companies will gather all of the information and determine whose policy will pay for the damages. Call your insurance agent as soon as possible, regardless of who is at fault. Find out whether you're covered for this loss. Even if the accident appears minor, it is important that you let your insurance company know about the incident. If your friend purchased insurance from the rental agency then this loss should be reported to them. If no citations were issued then you should not expect to see an increase in your premiums.

You and your friend can comparison shop your policies by getting a direct insurance quote here.