that does not imply he is not running X in the slightest.
You ARE running X right??
normally vesa output only runs as root due to device permissions. but inside X, xv, xover or x11 outputs should be fine. there can be the opposite problem here, that root can't open a display on a X screen that isn't roots in the first place.
in x, as your normal user, run "mplayer -vo x11 file.avi" and see what happens. if that fails, see what outputs you actually have using "mplayer -vo help". if you compiled it from sourc,e it mightn't have been able to find the X hedaers it needs. if so, install the xfree86-devel rpms form your distro cd's and recomiple mplayer