I'm looking at a different outfit by AS with a similar shader arrangement and similar render settings and light conditions; I get the same issue.  Increasing the Raytrace bounces appears to remedy the image quality under IDL.  IMHO, the Material shaders are not correctly composed for an IDL environment.  My best recommendation is to remove the Fastscatter from the Alternate_Diffuse and reroute the Color-Math node connected to the Diffuse_Color to the Alternate_Diffuse.  Connect the Diffuse map to the Diffuse_Color connector and 0 the Diffuse_Value.  In my assessment, the Fastscatter node is not contributing anything useful to the math.