If a colleague on your team is planning to quit his job and this may cause bad feelings in your company, would you notify your boss?
I probably would not, because 1) I assume it is not your job to manage people (since you refer to your boss), 2) this is one of the best ways to undermine the trust between yourself and your other colleagues. If you're concerned about the company, go to your colleague, express your concerns (DO NOT try to talk him out of quitting -- it is his life), and politely ask him to notify the boss in due time.

[re. 1): i tend to keep to myself stuff that I learn accidentally, but which is basically none of my business]
