Going to assume that this persons iBook was working fine up to the point of replacing the hard drive.
Does the power ring show as green or orange when plugged into the iBook? (Is there power getting to the iBook)
When the top cover is removed (Tracker Pad) it had a tiny little power lead from the power button to the logic board. These are somewhat difficult to remove carefully.
I'd check to see if the tiny plug at the end of the power button lead is in tract. Check to see if the pins on the logic board are not bent or broken.
The final thing to try, is a replacement button or worse still... a replacement cover.
Personally, buttons are a lot cheaper to unscrew and replace.
Other possibilities, Had the bottom cover been removed?
If they removed the bottom cover, it means the man power lead that runs from the DC in to the logic board would have become unplugged. Check to see if that had been plugged back in correctly.