I thinbk it will depend upin what state the aborted upgraded left things in. He might be able to do a Repair of WinME, then he might not. Best to assume the worst and secure any wanted files off the HDD (e.g. by putting HDD into a nother machine as 2nd drive) before proceeding.
Alternatively, just buy a 2nd HDD for existing machine and install ME (or XP) to that (as long as its not a laptop).