That program is probably to old/not compatible for Tiger.
Here's a Java version from the same page where it recommends using Chunk-joiner for the MAC platform.
HJSplit for Java
JAxe is a Java clone of the HJSplit program; it allows you to split a large file into smaller pieces and then join them back into the original file. JAxe is a Java clone of the HJSplit program. Being written in Java, JAxe runs on any platform which supports it, such as Windows, Linux, Mac, Solaris, freeBSD.
Note: It says you gotta have Java Virtual Machine installed & enabled on your Mac.
http://www.geocities.com/JAxe02/
Here's the original page that your friend probably downloaded his PC version of HJ Spilt
http://www.freebyte.com/hjsplit/#hjjoin
Split&Concat
Here's another app that states it works with OS 10.3.9 and higher
http://www.xs4all.nl/~loekjehe/Split&Concat/index.htm
Hope this helps.