The typical OPW insures you, the owner. If there’s an accident while your friend (who meets the opw requirements) is flying the plane, then the insurance company will pay you for damage to the airplane. And if there’s a lawsuit, your insurance will defend you, and pay up to the policy limits if they loose. BUT, your friend has NO coverage. The insurance company reserves the right to sue your friend for their loss. And you are contractually bound to help them. Your friend has no liability coverage. Usually, insurance companies are happy to add a cfi as a ‘named insured’. That’s what you should ask for.