Since he has audio I'm assuming he is using the parking brake and wouldn't have gotten audio. From personal experience, it depends on the cable or device. I've never really had any luck with the iPhone unless I had my iPhone Jailbroken and used the application in Cydia that allowed better support for "unsupported" cables and forced the audio/video to output. If your have a jailbroken device you can give that a try or try another official apple cable. If you want to test to see if its the device and not the system in the car, you can trying hooking another AV device such as a video camera (that allows playback via composite OR similar device) and see if it gives you the same issue.