|When someone provides you with personal information, there is an implicit assumption that you are going to keep it private. There is no way that your former colleague could have ever known that, for some reason, you feel compelled to tell your former managers everything you learn about their current employees. |
If you go to your former managers and tell them what this individual is planning on doing, you are acting in an unethical, unfair, unkind manner, and you probably don't want to do that. As for your fears of being "blamed," the only thing you could be blamed for is questionable judgment if you breach your former colleague's confidence. Literally and figurative, this is really not your business.
In addition, what would happen if your former associate changes his mind and decides not to leave, but you have already told management about his planned move? You could create some big problems for him, and some even bigger problems for yourself.