After the fight with Bracus my friend's client crashed.
Whenever I invited him to the game, he crashed.
We verified game the game cache and nothing.

We had to try all kinds of weird and nonsensical stuff people suggested in the steam forums.
Changing the session to invite only (isntead of friends only), sending items to the homestead, loading previous saves... Jesus what a pain it was... I don't even know what we did, but something fixed and he was able to join me without crashing.