Money Laundering regulations - High Value Dealers

A client has accidentally accepted cash of more than E15,000 for goods he has sold, valued at about £22,000. This appears to be a one off transaction, and the client has not accepted more than E15,000 cash for any other transaction. The client accepts cash for some small transactions, less than E15,000. I believe that the client is honest, and has not tried to conceal these transactions.

I have had a brief look at MLR8 and 9, which suggests that he should of registered as a high value dealer. As the client will not accept cash in excess of E15,000 again, does he have to register? What do I and the client have to do?