J is definitely more batteries-included than the open source K implementations.

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.

J is also way harder to learn in my opinion due to it having many times more operators. K2 and Q also have official books.

K is the easiest popular array language due to its array model and its commitment to simplicity. Gets you into the habit of writing out strings of symbols instead of names, and although it holds your hand less, I'd say it's great as a beginner language. With the existence of new docs like the oK docs(https://github.com/JohnEarnest/ok/blob/gh-pages/docs/) and the wiki(https://k.miraheze.org/), it is getting much easier to learn K.

I'd recommend APL2 and the GNU APL implementation instead. The reasoning is you can follow through the excellent Ken Iverson Turing Award paper [0], and, while using non-ascii is a small upfront cost, it IMO makes array languages much more readable.

[0]Notation as a tool of thought, https://doi.org/10.1145/1283920.1283935

I would recommend Dyalog APL over GNU APL, it is much more modern.

That's proprietary. And modern isn't an advantage in itself, unlike open-source for someone requesting open-source.

I know it's proprietary, it isn't my language of choice, but it's a lot better than GNU APL.

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.