Seems like a familiar story. Here's what this sounds like to me: the customer shuts down the computer and then cuts wall power by unplugging it from the wall as some misguided and under-informed environmental articles recommend. He/she also may be cutting power to the computer using a power strip, power "center," or battery backup device (UPS).
The results of any of those overwork the computer's internal backup battery, sometimes called the "PRAM" battery (equiv of the CMOS battery in a Windows computer) that was intended to maintain settings for only brief periods when the computer is unplugged for moving or maintenance. Assuming the computer remains on wall power the rest of the time as designed, a trickle current bypasses the power supply to preserve settings when the computer is turned off but plugged in; the battery does little or no work. If wall power is constantly cut, the battery must work much more than was intended and its life can drop from years to months.
Note: the difficulty in replacing the PRAM battery varies by PowerMac model from a 5-minute job to one that requires a major teardown, torque wrenches and thermal paste. Can you please tell us the exact model of PowerMac? There are three generation of PowerPC iMacs between 1998 and 2005. The second one, the flat-panel G4 iMacs, are the ones that require a serious teardown. If that is your client's model and he/she has been breaking its connection with wall power every day, your client may find that following misguided advise to save a tiny bit of electricity can prove to be a financially burdensome action.
If we know the exact kind of PowerPC iMac we can refer you to illustrated instructions for changing the battery.
