The G&L systems (Pic900 family, MMC family) run on compiled code. The only code that resides on the processor is compiled HEX code. You can upload and save this file for the purpose of downloading to another system, but you can't look at the ladder without the source file.
You might have one chance...if there is the optional FLASH RAM installed, the previous programmer might have stored his source code there. Otherwise you are out of luck. Try and find the original source disks, or his original PC if possible.
A word of caution, the battery back-up resides in the CSM module, not in the CPU module. If you remove the CPU from the rack for more than a day you will loose your program. There is a capactitor onboard the CPU that will retain the program for short periods, such as while replacing the battery.