Perhaps one of you has def_one_file='' and one of you has def_one_file='1'?
The def_one_file setting makes certain window/buffer related settings behave a little differently, at least from the user's perspective. For example, changing the font for a "file" actually changes it for the window, so if def_one_file='' then it changes the font for all files, but if def_one_file='1' then it changes the font only for the file in the current window.
Not complaining, just explaining.