IF ( and I wouldn't touch this with the proverbial ten foot pole) you take this on, get a retainer.  A big one.

I second the motion of sending him off to a tax attorney.

Your client *may* have been hoodwinked into signing a return falsely claiming a dependent.  I have clients I could do that to as they do NOT even look at what their return says.  But your client is responsible for what he signed (either paper file or efile).