It would be easier to diagnose what your neighbor was getting if we had his model but it sounds as if what he was doing was effecting the same as pushing an opening for a window (or a mortise) and this happens when the surface being pushed is bordered by at least one other face. You can see that here.
But if you pull the face in the opposite direction, it looks like it is a solid.
However orbiting around to the other side would show it is missing a face just as the first shot is.
This is perfectly logical behavior. If it didn’t work that way, you wouldn’t be able to cut openings as for windows or mortises.
As for the front and back faces, since SketchUp is a surface modeler, faces need to have fronts and backs. Think of it as building with AC grade plywoo. Keep the C grade faces on the inside of the project and the A-grade or front faces out. There are a number of different reasons to maintain proper face orientation. Since Push/Pull extrudes along a vector and vectors by definition have direction (positive or negative), the face orientation comes into play there. Other reasons for considering face orientation include the use of rendering applications. Many of them won’t render back faces even if they have materials applied to them.
An efficient workflow includes correcting face orientation when back faces show in 3D shapes. Keep up on it as you go and reduce the amount of work you’ll need to do later.