You'd be better off to just paint over what the friend did with an oil based primer, and then paint over the primer with whatever paint you want. Removing the oil based paint simply isn't very feasible.
What I'd do with what you have is use a piece of sandpaper to remove the roughness on the brown paper patch, and then repair that brown patch.
The best way to repair it is to apply strips of self adhesive fiberglass mesh drywall joint tape across the patch and run it up onto the painted wall a few inches on each side. Then, paint with white wood glue diluted with enough water to make it into a paintable consistancy. As the glue dries, it'll bond the fiberglass mesh to the drywall, thereby replacing the strength of the missing paper. Apply a second layer of fiberglass mesh strips running perpendicular to the first layer, and paint over them with diluted white wood glue as well. Now, mix up a bit of all purpose drywall joint compound (you may want to thin it by mixing in some water if it seems hard to spread easily) and apply that over the fiberglass mesh. Use a bright light held close to the wall, but some distance from where you're working to cast critical lighting on your work area to make it look rougher than it really is. That will give you a better idea of where to add joint compound and where to remove it to get a smoother surface. Once it looks OK under critical lighting, it'll look great under normal lighting, and you can prime and paint.