Apologies for the wait @Valorean, I haven’t received a response yet, though I have nudged the development team a further time. Hopefully there may be something else we can try 
I’ll be in touch as soon as I hear back.
In the meantime, if your friend is able to try reinstalling Steam, and then a clean reinstallation of Vermintide 2 this may well be worth trying - and if this fails it’s another potential solution we can then rule out.