If they installed VisualEditor master, than that is bound to fail because it requires 1.24alpha (as all extensions do unless otherwise noted).

There is a REL1_23 branch[1] which requires 1.22 or higher (not 1.24alpha).

It seems however that the custom 1.23 download made available is packages from some arbitrary version not documented anywhere and that version contains "wfUseMW( '1.24wmf6' );" in its code.

I suggest whatever hacks were applied to produce this tarball are applied to REL1_23 following regular code review. If it's based on master instead of REL1_23, we can fast-forward REL1_23 to master and apply the fixes then.

Either way whatever we promote as 1.23-compat should be git and go through review. And when it does, it should be fixed to not require wfUseMW( '1.24wmf6' ); which is a bit silly for code explicitly made for 1.23.

[1] https://github.com/wikimedia/mediawiki-extensions-VisualEditor/tree/REL1_23
[2] https://www.mediawiki.org/wiki/Extension:VisualEditor

I just downloaded the latest tarball from https://www.mediawiki.org/wiki/Special:ExtensionDistributor?extdist_extension=VisualEditor&extdist_version=REL1_23&extdist_submit=Continue, and I see wmfUseMW( '1.22' ) in it.

This sounds like the creation of the temporary tarball was very broken, but thanks to Kunal's work on ExtensionDistributor this should now be moot. Sorry for the troubles.