As long as the ESN is good (not reported stolen or lost) then it sounds like this person just had no idea what he was doing rooting, and probably screwed something up.
When you shut the phone off, boot back up holding power + x. What do you see? A triangle / exclamation mark and an android guy or something else? (I'm trying to figure out if you've got a custom recovery, which would be convenient).
If it says "ClockworkMod Recovery ..." or "SPRecovery ..." then you've got a custom recovery and you're not out of luck yet.
If that's the case, you can unroot (if it's still rooted, or keep it rooted

) in a few simple steps.
Don't lose hope! We're pretty good at getting people back from much worse than a few force closes here and there!