Given that AberMUD II was pretty widespread in the UK at the end of the 1980s, there isn't much source code still available. After some searching I came across only two complete distributions: A very late version, substantially reworked by one Richard Salz, that called itself AberMUD III; and an early version, allegedly distributed on the "X2 Support Group Game Development Archives", whatever that was.
Unfortunately, despite being what I was looking for at the time, the contents of this latter archive were in a pretty sorry state. It looked like it had been packed with an old MS-DOS archiver program, as all the filenames were truncated to 8.3, and all the directory information had been lost, causing some files with the same name in different directories to overwrite each other. Comparing against the later version, it also looked as if long lines had somehow been deleted from several source files.
Armed with these two versions, I decided to see if I could get the earlier and more historically interesting one to run, using the later version as a guide for any missing pieces. The resulting source code can be downloaded from Abermud2's GitHub page. I've built and run it under Ubuntu Linux and Cygwin (a Unix-like environment for Windows), and it should work under any other modern Unix, Linux or related system.