My interpretation of this as a manager is that this developer was probably a creative thinker with a decent track record who got stuck going down a bad path on this project, and nobody paid attention or intervened until it was too late. They were also probably pretty junior but perhaps had some past accomplishments that made them appear less likely to make this kind of mistake. Once it was in production, the developer very well may have been "stuck" with it (i.e., unable to get permission to scrap it and redo it, since it was technically working and solved some business problem).

Given the team dynamics and lack of involvement from this person's manager, I wouldn't move to fire them. I'd move to rethink the entire team, admonish the manager, and possibly remove them. The team itself wasn't working, and this was a symptom: someone had a bad idea, pursued it for too long, nobody did enough to stop it, and then they couldn't go back.

This is a classic consequence of a manager who has stopped paying attention to their own team. The team was most likely also overburdened with too many tasks, which is why everyone was working on something separate and independent and nobody knew what anyone else was doing. In reality this developer shouldn't have been given a project like this without being paired with a more senior engineer to supervise it, but that would cut down on the number of story points the team could get through and would thus be discouraged in a dysfunctional environment.

If he was in your team it would be your fault. I think you just need a healthy balance of senior/juniors on the same codebase.

As OP said they were siloed from each other and he definitely needed some mentorship. I've seen devs like that turn to incredible coders just after a couple of months of pair programming.

Yeah that is a great point, I would have never let it get to this point.

That is 100% a pet peeve of mine: Places that hire perfectly capable jr. engineers and then fail to give them the support they need.