You won't be able to make out most small marks on a vehicle from a camera at that distance & quality. Just remember, you did not do anything wrong. HE was the one who may or may not have caused that damage through no fault of your own. I wouldn't be surprised if you hear nothing more from this guy. Sending him to your insurance company right off the hop may just look like you are getting them to bully him into giving up. If he does want to talk more, simply explain that you cannot be held responsible for how the equipment you provide is used, and if it is misused, you are not liable.
If he genuinely thinks you are liable, he will take further action, at which point you direct to insurance. Otherwise he will probably forget in a few months and probably keep washing at your facility without any further incident. Just don't give in when you didn't do anything wrong and no compensation is due. This will spread faster than wildfire and you will probably have more claims of this nature if people know that you will provide compensation if people make enough noise.