Is internal audit mandatory?

I have been assisting one of my clients for the last year or to implement an ISMS. We are now at the stage where an internal audit (stage 1)has to be conducted. The question that I need to ask is this: The client does not have an Internal Audit Department. Would it be possible to assist the client in doing a self-assessment instead of the stage 1 audit? Is the internal audit mandatory for final certification or would a self-assessment serve the same purpose? A tentative date for the Management Review has already been set and the client is also in the process of looking for a Certification Auditor to conduct the Stage 2 Audit.