I start by stating that I know neither party in this dispute and I'm not taking sides on this though one often gets a general "feeling" about disputes. IMO Common sense is often the best guide in these situations.
Why would the customer make an issue if there isn't one? If the bass isn't defective as @Joseph kaye suggests what does @ADAM ROBERTS have to gain by bringing it to the attention of this forum? It also appears that at least one other BCer has unresolved issues with one of @Joseph kaye's projects.
If I were in the position that my reputation as a luthier was at risk I would do my damnedest to ensure that my customer is happy. That's what a reputable trader would do IMO.
If the situation is as stated I'm not clear why additional costs were incurred. The cost of repair (assuming there are no defective parts) is time. Compared to the loss of even one customer, which I would expect to be hundreds of pounds if not more, it's unclear why the luthier wouldn't make good on any "guarantee" .
If a new neck is the solution then consider replacing the neck. If, as @Joseph kaye, suggests, there is nothing wrong with the bass - take it back and return his money. Failure to resolve this essentially suggests the luthier values their reputation at the price of a defective neck. The value of good will in this dispute is far greater than the value of the instrument.
It would make sense to me to seek an amicable resolution to this matter rather than air dirty laundry on social media.
Make it right and put an end to this...