It would be interesting to see a writeup comparing J and the open-source implementation(s) of K. Which would be better for a complete novice at array programming?
There's also a wealth of documentation and books on J: J for C Programmers: https://www.jsoftware.com/help/jforc/contents.htm
Learning J: https://www.jsoftware.com/help/learning/contents.htm
And the New Vocabulary "Nuvoc" on the wiki: https://code.jsoftware.com/wiki/NuVoc
Edit: spelling.
[0]Notation as a tool of thought, https://doi.org/10.1145/1283920.1283935
If they really want something open source and good, J, open source ks, or BQN will be better. But if you're suggesting an APL dialect, Dyalog is much better than GNU despite being proprietary.