And why would he care anymore about it? The company failed him, so why would he be helpful for them anymore?

He did his job, he moved on, whatever happens there should not be expected to be his problem anymore.

That manager is toxic and should be reprimanded and/or fired. The employee certainly has no ethical obligation to do them any favors, but it could prevent that manager from abusing current and future workers in this way.

This is a situation where the interests of the workers aligns with the business's ability to succeed. Why not leverage that to try to improve working conditions there? Seems like a win-win for everyone but the toxic manager.

Because if the CEO is smart, they'll fire that manager before he does any more damage to the company, and maybe hire the developer back.

To help other people who end up at said manager/job.