Yikes. You should send his boss/ceo/investors a spite email telling the story about how he lost a worker who was working for 1/2 market rate by setting impossible deadlines.
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.
That is a horrendously bad suggestion.