10
OpenGL is the same in Linux, Windows and MacOSX, the difference is the windowing system. If you just want to get quickly into building multi-platform OpenGL applications, take a look at GLUT (open-source alternatives, Ubuntu comes with freeglut). GLUT is source-compatible between operating systems, so you can compile the same code, without modifications (theoretically), in Windows and Linux.
If you need something more elaborate, you should start using either GTK+ or Qt to build your application GUI. GTK+ has a GtkGLExt extension that creates a widget that contains a OpenGL context you can draw. Qt comes with the QtOpenGL module, which provides the QGLWidget widget.
hm, if your friend is familiar with opengl in windows, one solution could be to get him 'used' with cross-platform libraries, first in windows environment..
Make him download one of the frameworks (sdl,qt,glfw,glut.. etc. there are plenty these around (id recommend http://www.glfw.org/ , really easy and good framework with enough documentation in pdf format)) and port his old 'windows only code' to use the chosen library. After he is comfortable with the framework, switching to another platform will be less painful.