I would like to make a few points , This is only possible because your dad is the only one who has to deal with this program and also he only works on this one big complex thing which he knows really well .
Some automatic tools could help (although I doubt thay'll work on DBase III): Static analysis to see what's there, version control to start at the top and log your way through and be able to rollback to a previous working version.
But it's at the very least weeks of pain.