I don't think your friend is right. The map isn't rendering itself twice, it's rendering a low-quality 3D skybox as a background. This is common. Rockford does it as does paralake I believe.

It should be a non-issue as GMod is not even close to being GPU bound and the geometry is ridiculously simplified.

As for "nothing he could do", if it was a huge issue just force "r_3dky 0" and the 3D skybox is gone.