This is such a bad idea that I think it was already a mistake to post it to HN without using a throwaway account -- even if it is not a real-life inspired post,

This is such a bad idea that I think it was already a mistake to post it to HN without using a throwaway account -- even if it is not a real-life inspired post, your manager doesn't know that. I would delete the post. The main reason it's a bad idea is that if your coworker decides not to leave after all, you've just limited their growth at your co, perhaps permanently, and invisibly to them. I think the only way something like this could work is if you're able to find out the true largest source of unhappiness for the coworker, and then present it to your manager as a general problem (unattributed to your coworker) and say you're seriously worried it's starting to affect morale.