If you still want something lighter than XFCE you can try a window manager like dwm. It's really light and you can customize the entire things by modifying the source code. If you just wanna customize simple things (color, font, key binds, etc.) you can edit a neat little file called config.h
Hope this helps!
It is pretty cool.
In a way I did say if he wants something "lighter" than XFCE. I should have said "You have to start with something like XFCE, gnome, etc. Just to get your the general feel of linux" Then if you wanted something lighter you can try dwm.
He can easily install dwm, or any other wm and choose it from his log-in page. If he doesn't like it he logs out and chooses another ones that suits him well.