Welcome to the forum, First off dont panic, whats happened is the your mate has used a mode 1 formattted OS disc, and the OS has corrupted the nav computer.
Download and burn a v29 image from this website, make sure that the image is mode 2 and burn at x1 speed on a good quality cd.
then go through the upgrade procedure,
go to car,
open door,
goto boot,
eject map disc,
turn ignition to stage 1,
goto boot and put OS disc in,
got to drivers seat and follow instructions,
do not turn off ign or close the door.
hopefully that should clear the corrupted OS
Good luck
P.S use imagburn to write the cd
P.S if you have any problems, let me know and I'll make you a disc image up
