Unless he has the original style mother board where the CMOS battery is a NI_CAD soldered to the mother board (I have replaced many), or he can have a early replacement half size all-in-one type where the battery is part of the DALLAS REAL TIME CLOCK IC.
When a machine gets this old, what is inside of the control that is original minimizes to the BMDC/PWMNT, AUF and LCTLAUF, and the BOB boards.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)