sounds like hes either done 1 of 2 things.... knowing that the way to change the start button name is to edit the explorer.exe file with a hex editor... he has messed up the file that windows is reading
theres obviously a couple of ways he could have went about changing the file....
hopefully at the very least he started with a backup of explorer.exe
the common methods to change the name are either :
create a copy named something like explorer1.exe, etc........ then edit the new file..... then from dos replace the old file with the new one
if this is an NTFS system (cant see files from dos)...... he had to change the registry to point to the new file instead of the original
so how do you fix it........ simple..... if he made a backup of explorer.exe......... boot into dos and replace the file...... should be in C:\windows theres also another copy that must be replaced in C:\windows\system32\dllcache\
if he didnt make a backup....... youre hosed...... reinstall