Ok...here's the issue:
My friend is trying to extract a model out of a .bsp file. But every time he does so there are two layers...an outer and an inner. When i say outer and inner i mean that when you look at the model from an "Ariel View" there is the ground you walk on, and when you rotate the model and you are looking at in from the "Underside View" of the model there should be no bottom (like you are looking at the ground, yet you don't see any ground). But in my friend case....it's completely opposite. When he uses the "Halo CE BSP Tag Converter V2.5" there is both a ground in the "Ariel View" and the Underside View"....like someone used the "Shell Modifier" in 3ds Max and made the model double-sided. How can my friend fix this issue?

Thanks