So your friends iMac desktop
or
or
might have a bad battery?
Do you mean you really removed the lithium battery cell from an iMac? You have to remove the cover, logic board/mass storage chassis, and open the logic board. That's fricking complicated. And it's probably not to blame.
Possible problems:
-the power button is stuck. This is the most common problem with the CRT display models, and is easily remidied.
-the poewr suppply is on it's last leg, and is unable to supply enough power.
-A major component (hard drive, CDR drive, motherboard etc) is drawing excessive power thus pulling down a power supply rail and activating the crowbar.
Don't forget that the typical life of an iMac with decent use is about 3 years. If you really want to fix this, your best bet is to go to a Mac specific repair shop.