If the TGA from the client was not created with fields originally ( Which I assume was not ) then the footage settings for this TGA should be set no "No Fields".
Your composite settings should be set to fields.
What you should also make sure is that you are working in 3D mode, not 2D mode. From some strange reason 2D mode is NOT rendering in fields, only frames, so not matter what settings you set for fields in 2D you will only get rendered frames.
Change the composition to 3D in the settings menu then try render again and let us know.....