The short answer is yes; the long answer is usually. Like Marvin said, not all Linux/UNIX is the same - to run a program on OSX, it needs to have been compiled specifically for a particular platform. Much *NIX software is open-source, so you can compile it yourself, or use a repository that compiles it for you (like Fink or DarwinPorts). Commercial *NIX apps will only run on the specific platforms they were released for (usually Linux/x86 and Solaris). Whether your dad will be able to run his programs depends on what he actually wants to run.
You mentioned a MacBook Pro, which adds another little kink to the story - the Intel Macs are a different platform than PPC Macs. Programs compiled to run on Darwin/PPC won't run on Darwin/x86. Even for open-source programs, there's been a lag in getting everything tested and stable on Darwin/x86, and some of the larger projects still have bugs to work out. So at the moment, there's generally better support for *NIX apps on PPC Macs - but that will change at some point in the near future.