Dear Rajan, As you have not mentioned in which year the assessee has taken the service tax registration certificate for the services he provide.
For your clarification, when a person/firm/company or any other assessee who opts for services tax registration for the services they serve can obtain the service tax registration certificate on the following 2 basis.
1) When the assessee whose gross receipts cross 10 lakhs in the preceding year can opt for the registration and has to pay the tax as prescribed.
2) The assessee has another option i.e., taking the certificate on optional basis. in this way the assessee has the pay the service tax & file the returns to the concerned authorities even though he doesn't crosses the gross receipts as prescribed by the service tax authorities.
For taking the service tax registation on the option 2 basis, then the assessee has to give a declaration to the superindent for paying the service tax and filing the return even though he doesn't crosses the gross receipts.
Please verify whether, your client has given this declaration to the authorities or not at the time of registation and also your client is ready to pay the tax or not.