rental car chaos: a coverage conundrum

Last weekend, my buddy Jake rented a car for our road trip. Everything was chill until we stopped for lunch and came back to find a mysterious dent on the passenger door. No note, no witnesses, nada. Jake swears he declined extra coverage at the rental place, but now he's freaking out about what exactly his credit card covers. Honestly, I never really thought about this stuff before...anyone been through something similar?