Napa should never have been signed, however he is now a big problem for the club because his trajectory matches the narrative that we are the place where good players come to see their careers die. It isn't the clubs fault, but it is the clubs problem.
I'd suspect it is at least partly why Napa continues to get game time, to try and get some decent performances out of him to both kill the unfavourable narrative and because the club would be wanting him to get a decent contract somewhere else, even England, just to try and kill off this narrative. Sacking him would be tempting in the sense that it would support the idea that the problem is more with the player, not the club, but it comes with some serious dangers. Unlike the Vaughan matter, it isn't necessarily clear that Napa has dominant role in the indiscretion, and if they are going to fire him the club is going to need to show that this is the case, else they risk their actions backfiring in that it ends up strengthening the narrative that we are the club that kills careers.