I remember a client asking for vtiger once. I downloaded the source, and didn't even get to installation before I fired the client.

I really, really feel for you on this one.

The only thing I can say is: Make a beachhead of clean, working code. Slowly work your way out. Make it very clear to this client how much of a favor you're doing them, and give them meaningful status reports (even if all you did is rewrite the glue between to pieces of code).