Were these published documents, which reviewers could independently obtain and verify?
That doesn't stop some of the people at WP from reverting everything. Which means OP's friend is left to either leave it reverted, or trawl through the various arcane dispute resolution / meta pages, arguing their case, building consensus, to eventually get people saying it should be left in. Or saying it should be left out because those meta pages sometime feel as random as tossing a coin.
Unsurprisingly this optimizes for people who tolerate vast amounts of meta bullshit, and not people who know what they're talking about and know what the good sources are.
But facts ("Here's the catalog number in the New York Public Library, and how to request it via interlibrary loan.") shouldn't require consensus.
Especially for cases like that - if someone has more 'sway' on Wikipedia than you, they can (and will often) just say something like "Thanks for the source - I'll verify and if it says what you think it says, I'll add it in to the article." Then do nothing, ever again. In fact, unreliable citations, or citations that don't actually say what the citer thinks they say, that can be easily checked online are far more acceptable on Wikipedia than citing a book.
Inertia like this leaves useless pages like "Oplomachi" (https://en.wikipedia.org/wiki/Oplomachi) live way after they should have been merged, as well as leading to the deletion of useful pages/sections/references.
The major articles, which are more likely to get attention, tend to be less dysfunctional than niche interests. That said, there are of course far more articles relating to niche interests than there are major articles.
At that point it was too late... anything he did was on some tickler list and waging wiki-war was not worth the time investment.
Then someone can link form Wikipedia to Wikia.
Wikia isn't just for TV shows and games. For example, the text editor Vim has a Wikia section: vim.wikia.com
There's just no justification for that. Even if a policy required all references to be available via the internet, the author fulfilled that requirement.
If this was allowed, and Donald Trump is putting up his documents on the internet and referencing them on his wikipedia page would you allow it? It can do a lot of damage and people and organizations do this all the time and every day.
This is the kind of behavior the policy exists to prevent. Its well known that it isn't perfect Please read - https://en.wikipedia.org/wiki/Wikipedia:Conflict_of_interest
Edit: ... their own unpublished stuff.
He attracted the ire of some wackjob when he referenced printed materials. Given his somewhat unique position, he had some stuff digitized and posted, and returned to Wikipedia a few months later to find that almost everything he did was reverted.
I'm glad that people put up with the nonsense and contribute to Wikipedia... but what a shit experience.